Seminar 4

Carbohydrates

3

Saccharides (glycids)

are polyhydroxyaldehydes, polyhydroxyketones, or substances that give such compounds on hydrolysis

Definition

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POLYSACCHARIDES

polymeric

Give monosaccharides when hydrolyzed

GLYCANS

Basal units

MONOSACCHARIDES polyhydroxyaldehydes polyhydroxyketones

OLIGOSACCHARIDES

2 – 10 basal units

GLYCOSES (sugars) water-soluble, sweet taste

Don't use the historical misleading term carbohydrates, please. It was primarily derived from the empirical formula Cn(H2O)n and currently is taken as incorrect, not recommended in the IUPAC nomenclature (even though it can be found in numerous textbooks till now)

Classification

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• occur widely in the nature, present in all types of cells

– the major nutrient for heterotrophs

– energy stores (glycogen, starch)

– components of structural materials (glycosaminoglycans)

– parts of important molecules (nucleic acids, nucleotides, glycoproteins, glycolipids)

– signalling function (recognition of molecules and cells, antigenic determinants)

Saccharides

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are simple sugars that cannot be hydrolyzed to simpler compounds

Aldoses Ketoses Simple derivatives (polyhydroxyaldehydes) (polyhydroxyketones) modified monosaccharides

are further classified according to the number of carbon atoms in their chains:

glyceraldehyde (a triose) dihydroxyacetone tetroses tetruloses pentoses pentuloses hexoses hexuloses heptoses … heptuloses …

deoxysugars amino sugars uronic acids

other simple derivatives

alditols glyconic acids glycaric acids

Trivial names for stereoisomers

glucose (i.e. D-glucose) fructose (i.e. D-fructose) L-idose L-xylulose, etc.

Systematic names (not used in biochemistry) comprise trivial prefixes according to the configuration: e.g., for glucose D-gluco-hexose, for fructose D-arabino-hexulose

Monosaccharides

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Secondary alcoholic groups CH-OH in monosaccharides are stereogenic centres. Monosaccharides are chiral compounds and, therefore, most of them are optically active

Stereogenic centres are mostly carbon atoms that bind four different groups; those atoms are often called "asymmetric" carbon atoms

If there are more (n) stereogenic centres in the given molecule,

the maximal number of stereoisomers equals 2n

Each of those stereoisomers has its enantiomer (mirror image) so that there will be a maximum of 2n / 2 pairs of enantiomers

Stereoisomers that differ from the particular pair of enantiomers are diastereomers of the pair

In contrast to enantiomers, diastereomers differ in their properties and exhibit different values of specific optical rotation

Stereoisomerism in monosaccharides

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are structural formulas that describe the configuration of particular stereoisomers When a plane formula of an aldose with four stereogenic centres is drawn anywhere

an hexose

it is necessary to see a spatial arrangement of the atoms and assess it according to the established rules:

• the least number carbon (carbonyl group in monosaccharides) is drawn upwards

• the carbon chain is directed downwards then on each stereogenic centre

• the bonds to neighbouring carbon atoms written above and below are projected from beneath the plane of drawing (the carbons are behind the plane)

• the horizontal bonds written to the left and right are projected from above the plane of drawing, they are in front of plane

Fischer projections formulas

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Assigning configurations D- and L- (from Latin dexter and laevus) at stereogenic centres is carried out

by comparison with the configurations of D- and L-glyceraldehyde

Without changing the configuration,

Fischer formulas may only be turned 180° in the plane of the paper.

Monosaccharides are classified as D- or L-sugars according to

configuration at the configurational carbon atom – the chiral

carbon with the highest numerical locant (i.e. the asymmetric

carbon farthest from the aldehyde or ketone group):

D-aldose L-ketose

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D-allose D-glucose

What is that?

D-mannose L-glucose

Enantiomers, diatereomers, epimers

• L-glucose is enantiomer of D-glucose because of

having opposite configuration at all centres of chirality

• Are there, among the following sugars, some diastereomers

of D-allose that are not epimers of it?

• Is there any epimer of D-mannose?

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Stereogenic centres in molecules of monosaccharides are the cause of their optical activity Solutions of mono- and oligosaccharides turn the plane of polarized light Optical activity is measured by using polarimeters and usually expressed as specific optical rotation [α]D

20.

Dextrorotatory substances are marked (+), laevorotatory (–)

Configurations at stereogenic centres other than configurational carbon cannot be deduced from the assignment to D- or L-sugars.

Unfortunately, configurations of several most important monosaccharides have to be remembered

There is no obvious relation between the assignment D- or L- and either the values or direction of optical activity

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D-glyceraldehyde

D-erythrose D-threose

D-ribose D-arabinose D-xylose D-lyxose

D-allose D-altrose D-glucose D-mannose D-gulose D-idose D-galactose D-talose

D- Aldoses stereochemical relations

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D-(–)-erythrose D-(–)-threose

D-(–)- arabinose D-(+)-xylose D-(–)-lyxose

D-(+)-allose D-(+)-altrose D-(–)-gulose D-(–)-idose D-(+)-talose

D- Aldoses optical rotation

D-(+)-glyceraldehyde

D-(–)-ribose

D-(+)-glucose D-(+)-mannose D-(+)-galactose

(+) dextrorotatory

(–) laevorotatory

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D-(–)-erythrulose

D-(+)-xylulose

D-(+)-psicose D-(+)-sorbose D-(+)-tagatose

D- Ketoses stereochemical relations

dihydroxyacetone

D-(–)-fructose

D-(–)-ribulose

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Cyclic forms of monosaccharides

Monosaccharides (polyhydroxyaldehydes and polyhydroxy- ketones) undergo rapid and reversible intramolecular addition of some properly located alcoholic group to carbonyl group

so that they form cyclic hemiacetals

Monosaccharides exist mainly in cyclic hemiacetal forms, in solutions the acyclic aldehydo- or keto-forms are in minority.

al-D-glucose a hemiacetal, pyranose ring

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In this way, six- or five-membered rings can originate.

In pyranoses, there is the tetrahydropyran (oxane) ring, tetrahydrofuran (oxolane) ring

in furanoses.

In the acyclic forms, carbon of the carbonyl group is achiral, but this carbon becomes chiral in the cyclic forms. Two configurations are possible on this new stereogenic centre

called anomeric (or hemiacetal) carbon so that the

cyclization results in two epimers called α or β anomers:

α-anomer β-anomer

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• the configuration of - anomer is the same as the configuration at anomeric reference carbon

• in monosaccharides comprising five and six carbon atoms (pentoses and hexoses, pentuloses and hexuloses), the anomeric reference carbon is the configurational carbon α-anomers in Fischer formulas of D-sugars have the anomeric hydroxyl localized on the right

• the configuration of β-anomers is opposite, the

anomeric hydroxyl is written on the left in Fischer formulas of D-sugars

The hemiacetal hydroxyl group is called the anomeric hydroxyl

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In solutions, all five forms of a hexose or hexulose occur; the cyclic forms usually prevail

E.g., in the aqueous solution of D-glucose equilibrated at 20 °C, there is approximately 62 % -D-glucopyranose, 36 % -D-glucopyranose, < 0.5 % -D-glucofuranose, < 0.5 % -D-glucofuranose, and < 0.003 % aldehydo-D-glucose.

If D-glucose is crystallized from methanol or water, the pure α-D-glucopyranose is obtained; crystallization of D-glucose from acetic acid or pyridine gives the β-D-glucopyranose. These pure forms exhibit mutarotation, when dissolved:

α-D-Glucopyranose just after dissolution exhibits [α]D20 = + 112°, the β-form

[α]D20 = + 19°. After certain time period, [α]D

20 of both solutions will settle at the same equilibrium value of + 52°. This change can be explained by opening of the cyclic homicidal to the acyclic aldehyde. which can then recyclize to give either the α or the β form till an equilibrium is established.

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Epimers – are those diastereomers that differ in configuration at only one centre of chirality, they have the same configuration at all stereogenic centres except one.

Don't confuse:

Enantiomers (optical antipodes) – stereoisomers that are not superimposable mirror images of each other, the configurations at all stereogenic centres are exactly opposite. All their chemical and physical properties are the same but the direction of optical rotation.

Anomers (α or β) represent a special kind of epimers, they have identical configuration at every stereogenic centre but they differ only in configuration at anomeric carbon atom.

Diastereomers – stereoisomers that are not enantiomers of one another. They have different physical properties (melting points, solubility, different specific optical rotations) so that they are viewed as different chemical substances.

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Haworth projection formulas

α-D-glucopyranose

Fischer projection Haworth projetion (the usual basal position)

– the rings are projected as planes perpendicular to the plane of drawing,

– carbon atoms of the rings and hydrogens attached to them are not shown,

– each of the formulas can be drawn in four positions, one of which is taken as the basal position (used preferentially)

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Rules for drawing Haworth projection formulas (the basal position):

C

1

OH

pyranose ring of a hexose

C

1

OH

furanose ring of a pentose

C C

2

OH

furanose ring of a hexulose

– The anomeric carbon atom (C-1, in ketoses C-2) on the right;

– oxygen atom in the ring is "behind", i.e. carbon atoms are numbered in the clockwise sense;

Then, – hydroxyl groups and hydrogens on the right in the Fischer projection are down in the Haworth projection (below the plane of the ring), and conversely, hydroxyls on the left in Fischer formulas means up in Haworth formulas;

– the terminal –CH2OH group is up for D-sugars (for L-sugars, it is down).

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α-D-glucopyranose can be drawn in four different positions:

The basal position: Position obtained by rotation of the "model" round a vertical axis

O

Positions obtained by tilting the „model” over: because the numbering of carbons is then counter-clockwise, the groups on the right in Fischer projection as well as the terminal –CH2OH are up in those Haworth formulas:

or

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al-D-glucose α-D-glucopyranose β-D-glucopyranose

β-D-glucofuranose α-D-glucofuranose

Four different cyclic forms of glucose

(all are depicted in the basal position)

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Four different cyclic fructose forms

α-D-fructofuranose β-D-fructofuranose

keto-D-fructose

β-D-fructopyranose α-D-fructopyranose

(all are depicted in the basal position)

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Conformation of pyranoses

α-D-glucopyranose-4C1 β-D-glucopyranose-4C1

The chair conformation of six-membered rings is more stable than the boat one. From two possible chair conformations, that one prevails, in which most of the voluminous groups (-OH, -CH2OH) are attached in equatorial positions.

steric hindrance

boat conformation 4C1-chair conformation 1C4-chair conformation

E.g., conformations of β-D-glucopyranose:

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D-fructose

Reduction of monosaccharides results in formation of

D-glucose D-glucitol

D-mannitol

alditols (sugar alcohols):

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Oxidation of monosaccharides

a glyconic acid (aldonic)

an aldose

a glycaric acid (aldaric)

a glycuronic acid (uronic acid)

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D-Glucose

(dextrose, grape sugar) is in the form of polysaccharides (cellulose, starch, glycogen) the most abundant sugar in the nature

Important monosaccharides

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D-Galactose

is the 4-epimer of glucose.

It occurs as component of lactose in milk and in dairy products (hydrolysis of lactose in the gut yields glucose and galactose), and as a component of glycoproteins and glycolipids.

D-Galactose β-D-Galactopyranose

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D-Ribose

β-D-ribofuranose β-D-ribopyranose

is the most important pentose – a component of nucleotides and nucleic acids:

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D-fructose

D-Fructose

(laevulose, fruit sugar) is the most common ketose, present in many different fruits and in honey. A considerable quantities of this sugar are ingested chiefly in the form of sucrose

β-D-fructofuranose β-D-fructopyranose

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Simple derivatives of monosaccharides

Esters

base

nucleoside 5´-phosphate fructose 1,6-bisphosphate

glucose 1-phosphate glucose 6-phosphate

with phosphoric acid are intermediates in metabolism of saccharides, constituents of nucleotides, etc-

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Deoxysugars

Deoxyribose (2-deoxy-β-D-ribose) is a constituent of nucleotides in DNA

L-Fucose (6-deoxy-L-galactose) is, e.g., present in some determinants of blood group antigens, and in numerous glycoproteins

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Amino sugars

are important constituents of saccharidic components of glyco- proteins and glycosaminoglycans.

N-acetylgalactosamine α-D-glucosamine N-acetylglucosamine

glucosamine (2-amino-2-deoxy-D-glucose)

fructose

CH–

CH=O

NH2

CH–OH

CH2–OH

HO–CH

CH–OH

CH–OH

CH2–OH

HO–CH

CH–OH

C=O

CH2–OH

The basic amino groups –NH2 of amino sugars are nearly always "neutralized“ by acetylation in the reaction with acetyl-coenzyme A, so that they exist as N-acetyl-hexosamines. Unlike amines, amides (acetamido groups) are not basic.

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HC=O

HO–CH

HC–OH

CH2–OH

NH2–CH

HC–OH

C=O

COOH

C H 2

HC–OH

HO–CH

HC–OH

CH2–OH

NH2–CH

HC–OH

CH3

C=O

COOH

is an aminononulose (ketone) as well as glyconic acid, 5-amino-3,5-dideoxynonulosonic acid.

It originates in the cells by condensation of pyruvate (in the form of phosphoenolpyruvate) with mannosamine:

Neuraminic acid

mannosamine

pyruvate

neuraminic acid

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Sialic acids are constituents of saccharidic components of glycolipids (gangliosides) and glycoproteins

Sialic acids

is the group name used for various acylated derivatives of neuraminic acid (N- as well as O-acylated)

The most common sialic acid is N-acetylneuraminic acid:

neuraminic acid sialic acid N-acetylneuraminic acid

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Glycuronic acids (uronic acids)

D-galacturonic acid D-glucuronic acid

D-Glucuronic acid originates in human bodies by oxidation of activated glucose (UDP-glucose). It is a component of glycosaminoglycans in connective tissue and some hydrophobic waste products and xenobiotics are eliminated from the body after conjugation with glucuronic acid.

D-Galacturonic and L-iduronic acids occur also as components of numerous glycoproteins and proteoglycans.

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Glyconic acids are polyhydroxycarboxylic acids obtained by oxidation of the aldehyde group of aldoses. E.g., glucose gives gluconic acid:

In the body, glucose (activated to glucose 6-phosphate) is dehydrogenated in the enzyme-catalyzed reaction to phosphogluconolactone that gives phosphogluconate by hydrolysis. This reaction (the initial reaction of the pentose phosphate pathway) is very important as a source of NADPH.

D-gluconic acid gluconate

1/2 O2

glucose 6-phosphate

– P

D-glucono-1,5-lactone

– P

D-glucono-1,4-lactone

– P NADP+ NADPH+H+

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L- Ascorbic acid

It is a weak diprotic acid (endiols are acidic), which has outstanding reducing properties. It can be very easily oxidized, to dehydroascorbic acid, namely in alkaline solutions.

Ascorbate acts as a cofactor of several enzymes and a powerful hydrophilic antioxidant. It is essential only for humans, primates, and guinea pigs.

– 2H – 2H

L-gulose L-gulonic acid L-gulono-1,4-lactone L-ascorbic acid dehydro-L-ascorbic acid

(2,3-dehydro-L-gulono-1,4-lactone, vitamin C) is derived from L-gulonic acid.

Deducing of the structure of ascorbate:

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+ HO-CH3 – H2O

glycosidic bond

Glycosides

Cyclic forms of saccharides, relatively unstable hemiacetals, can

react with alcohols or phenols to form acetals called glycosides.

The hemiacetal hydroxyl group (the anomeric hydroxyl) on the anomeric carbon is replaced by an alkoxy (or aryloxy) group.

The bond between the anomeric carbon and the alkoxy group is called the

glycosidic bond or O-glycosidic bond, at need.

Similarly, glycosidic bonds can be formed by reaction with an amino group, N-

glycosidic bonds, or with a sulfanyl group, S-glycosidic bonds

Example:

α-D-glucopyranose methanol methyl-α-D-glucopyranoside

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Classification of glycosides

Hologlycosides are glycosides that give only monosaccharides by hydrolysis - O-glycosidic bonds bind various number of monosaccharides.

Oligosaccharides – consist of as much as approximately ten monosaccharides; the most common are disaccharides.

Polysaccharides comprise up to many thousands monosaccha- ride units bound through glycosidic bonds. Those units are either of the same kind in homopolysaccharides, or may be of several kinds in heteropolysaccharides.

Heteroglycosides in which nonsaccharidic components called aglycones or genins are linked to saccharides through glycosidic bond This bond may be not only O-glycosidic but also N-glycosidic or S-glycosidic.

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Disaccharides are the most common disaccharides, in which two monosaccharides are linked through glycosidic bond. There are two types of these sugars – reducing and nonreducing disaccharides.

Reducing disaccharides are formed by a reaction between the anomeric hydroxyl of one monosaccharide and a alcoholic hydroxyl group of another, so that this second monosaccharide unit retains its anomeric hydroxyl, the reducing properties, it may anomerize and exhibits mutarotation.

Their names take the form D-glycosyl-D-glycose (with specification of the glycoside bond).

Nonreducing disaccharides Both anomeric hydroxyl are linked in the glycosidic bond (called anomeric bond), neither unit has its anomeric hydroxyl. They cannot reduce Benedict's reagent and cannot mutarotate.

Their names have the form D-glycosyl-D-glycoside.

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Maltose

Reducing disaccharides

(4-O--D-glucopyranosyl-D-glucopyranose, malt sugar) is obtained by the partial hydrolysis of starch or glycogen. Two molecules of glucose are linked through (1→4) glycosidic bond, further hydrolysis results in only glucose. Maltose is laevorotatory. Crystalline maltose is the β-anomer and exhibits mutarotation, when dissolved..

β-maltose 4-O--D-glucopyranosyl-β-D-glucopyranose

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Isomaltose

may be viewed as a constituent of glycogen and amylopectin placed

at branching points of the long chains connected through α(1→4) bonds.

α-isomaltose 6-O--D-glucopyranosyl-α-D-glucopyranose

(1→6) glycosidic bond

6

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Cellobiose

(4-O-β-D-glucopyranosyl-D-glucopyranose) is obtained by the partial hydrolysis of cellulose. Two molecules of glucose are linked through β(1→4) glycosidic bond, further hydrolysis results in only glucose. Cellobiose is dextrorotatory.

4

β-cellobiose 4-O--D-glucopyranosyl-β-D-glucopyranose

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Lactose

(4-O-β-D-galactopyranosyl-D-glucopyranose, milk sugar) is the major sugar in human and cow's milk. Equimolar mixture of glucose and galactose is obtained by hydrolysis of β(1→4) glycosidic bonds. Lactose is dextrorotatory. Crystalline lactose is the α-anomer and exhibits mutarotation, when dissolved.

α-lactose 4-O--D-galactopyranosyl-α-D-glucopyranose

β

4

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1

2

β

α

Nonreducing disaccharides

Sucrose (saccharose)

(-D-fructofuranosyl--D-glucopyranoside, beet or cane sugar) is the ordinary table sugar. Both hemiacetal hydroxyl groups of fructose and glucose are involved in the (β2↔α1) glycosidic bond (called occasionally anomeric glycosidic bond).

Sucrose is dextrorotatory and cannot mutarotate. When hydrolyzed, an equimolar mixture of glucose and fructose results that is laevorotatory (invert sugar), because the anomers of fructose are stronger levorotatory than the dextrorotatory anomers of glucose.

sucrose -D-fructofuranosyl--D-glucopyranoside

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obtained X-ray structural analysis of crystalline table sugar

Real conformation of a sucrose molecule

Seminar problems answers

Q1. For each of followings structure decide whether the carbohydrates is: an aldose or ketose and tetrose, pentose or hexose.

(a) (b) (c)

Aldose Hexose

Ketose Hexose

Aldose Tetrose

Q2. Easy – memorising

Q3. Easy – memorising

Q4. The followings questions apply to the sugar A.

a) Ketose

b) Anomeric carbon

c) OH C4

d) This is β (D)

Q5. Use the following structures in answering the next questions.

a) L-sugars: B and G – last asymatric atom (the highest locant) is 4 (OH up means it is on the left)

b) α-anomers: D, E c) Reducing sugars: A, C, D, F, G d) Derived from the same sugar: A and F, E and H e) Deoxy sugars: Only C f) Names od „D” sugar: α- and β-D-fructofuranose

Q6. Easy – memorising

Q7. Easy – memorising

Q8. Which one of the followings would show mutarotation when dissolved in water? the following structures in answering the next questions.

Mutarotation: Only C (it has „free” anomeric carbon)

Q10. Which of the following sugars are reducing sugars?

Reducing sugars

Q9. Easy – memorising

Q13. A 4 g sugar cube (Sucrose: C12H22O11) is dissolved in a 350 ml teacup of 80°C water. What is the percent composition by mass of the sugar solution?

Given: Density of water at 80 °C = 0.975 g/ml

𝐶𝑝 =4 𝑔

4 𝑔 + 350 𝑚𝑙 ∙ 0,975 𝑔

𝑚𝑙

∙ 100%

Q11. Easy – memorising

Q12. Easy – see the lecture