A Level Chemistry

Question 1

A carbon compound P has the percentage composition 85.7 % carbon and 14.3 % hydrogen. Its relative molecular mass was found to be 56.

a.

i. Calculate its empirical formula.

ii. Calculate its molecular formula.

b. Write down the names and displayed formulae of all the non-cyclic isomers of compound P which have the following characteristics:

i. straight chain

ii. branched chain.

a.

i. we have the relative mass of carbon, Ar = 12.0, and the relative mass of hydrogen, Ar = 1.0

We assume that there are 100g of compound P.

thus, there are 85.7 g of C per 100g of compound and 14.3 g of H per 100g of compound.

The number of moles of C = `85.7 / 12.0` = 7.14 moles

The number of moles of H = `14.3 / 1.0` = 14.3 moles

The ratio between number of these atoms

C = `7.14 / 7.14` = 1

H = `14.3 / 7.14` = 2

Thus, the empirical formula is CH₂

ii. The relative molecular mass of this empirical formula is `12.0 + 2 xx 1.0 = 14.0`

We know that its relative molecular mass was found to be 56. Therefore, the number of units of the empirical formula is `56 / 14.0 = 4`

Thus, the molecular formula is C₄H₈

b.

i.

ii.

Question 2

He reacted ethane with chlorine in the presence of UV light by the following reaction:

`C _2H_6(g) + 2Cl_2(g) → C_2H_4Cl_2(l) + 2HCl(g)`

After doing this he found that 600 g of ethane gave 148.5 g of C₂H₄Cl₂.

A. How many moles of ethane are there in 600 g?

B. How many moles of 1,2-dichloroethane would have been formed if the yield had been 100 %?

C. How many moles of 1,2-dichloroethane are there in 148.5 g?

D. Calculate the percentage yield of 1,2-dichloroethane.

A. The number of moles of ethane = `600 / 30`= 20 moles

B. The ratio of moles between ethane and 1,2-dichloroethane is 1:1, as such, to obtain 100 % of yield, the moles of 1,2-dichloroethane = the moles of ethane = 20 moles

C. The number of moles of 1,2-dichloroethane in 148.5 g = `148.5 / 99` = 1.5 moles

D The percentage yield of 1,2-dichloroethane = `1.5 / 20 xx 100%` = 7.5 %

Question 3

He reacted ethene with chlorine in the dark by the following reaction:

`C_2H_4(g) + Cl_2(g) -> C_2H_4Cl_2(l)`

In this reaction 140 g of ethene gave 396 g of C₂H₄Cl₂.

A. Calculate the percentage yield for this reaction. Show your working.

B. There are isomers of the compound C₂H₄Cl₂. Draw the displayed formulae of the isomers and name them.

C. Choose from redox, substitution, elimination, addition and hydrolysis to give the type of reaction for this reaction

A.

The number of moles of ethene = `140 / 28` = 5 moles

The number of moles of C₂H₄Cl₂= `396 / 99` = 4 moles

The percentage yield of this reaction = `4/5 xx 100%` = 80 %

B.

C. The type of reactions for this reaction is addition which involves the formation of a single product from two reactant molecules.

Question 4

X can be manufactured by the oxidation of propene.

`CH_3CHCH_2 + O_2 → CH_2CHCHO + H_2O`

Name X and state the functional groups that are present.

The name of X is propenal

The functional groups that are present in the molecule are alkene and aldehyde

Question 5

a. Crotonic acid, C₄H₆O₂, is used in the manufacture of paints and adhesives.

State the empirical formula of crotonic acid.

b. The systematic name of crotonic acid is but-2-enoic acid.

Draw two displayed formulae for possible isomers of crotonic acid that retain the carboxylic acid functional group.

a. The empirical formula of crotonic acid is C₂H₃O

b. The displayed formulae for possible isomers of crotonic acid that retain the carboxylic acid functional group

Question 6

Z is a saturated hydrocarbon that is an isomer of pentene. The molecular formula of Z matches the general formula of the homologous series of alkenes.

Draw the skeletal formula of Z.

The general formula of the alkenes is C_nH_2n.
Z is a saturated hydrocarbon, which means that it must be an alkane. The only way that an alkane can have an alkene-like molecular formula is for it to be a cyclic alkane.
Since the alkene is pentene, this means that Z must be cyclopentane. Thus, the skeletal formula of Z is

Question 7

What is the identity of the functional group that is ringed in the molecule shown below?

A. Aldehyde

B. Ketone

C. Carboxylic acid

D. Alkene

The answer is B

A is incorrect because the carbon in the C=O of an aldehyde is bonded to a hydrogen and another carbon

C is incorrect because carboxylic acid has a -COOH functional group

D is incorrect because an alkene has a C=C functional group

Question 8

What is the IUPAC name of the following molecule?

A. 2-chloro-3-methylpentane

B. 3-methyl-4-chloropentane

C. 2-ethyl-3-chlorobutane

D. 2-chloro-3-ethylbutane

The answer is A

The naming rules begin with the longest carbon chain then counting the side groups and designate their position with the lowest number possible along the carbon chain

The -CH3 is a methyl group

The -Cl is a chloro group

B is incorrect because the numbers do not give the lowest combinations of the positions of the side groups along the chain

C and D are incorrect because the longest chain is 5 not 4

Question 9

Which of the following molecules are hydrocarbons?

A. 1 and 2

B. 1, 2, 3 and 4

C. 1, 2 and 3

D. 4 only