WAEC Chemistry Past Questions & Answers

81.

A white precipitate was formed when BaCl $_{2(aq)}$ was added to an aqueous solution
of a salt X. The precipitate dissolved in dilute HCl with rapid effervescence. Salt X is likely to contain

SO $_{4}$ $^{2-}$ ions

SO $_{3}$ $^{2-}$ ions

NO $_{3}$ - ions

SO $_{3}$ $^{2-}$ - ions

View answer & explanation

Correct answer is D

Effervescence is the formation of gas bubbles in a liquid by a chemical reaction. An example of effervescence is the release of carbon dioxide which bubbles as a gas from the liquid when limestone chips, which are composed of calcium carbonate, are added to dilute hydrochloric acid.

82.

Increasing the temperature generally

decreases the solubility of a solid in a liquid but increases the solubility of a gas in a liquid.

increases the solubility of a solid in a liquid but decreases the solubility of a gas in a liquid.

increases the solubility of both a solid and a gas in a liquid

decreases the solubility of both a solid and a gas in a liquid

View answer & explanation

Correct answer is B

-The solubility of gases in liquids decreases with increasing temperature. Conversely, adding heat to the solution provides thermal energy that overcomes the attractive forces between the gas and the solvent molecules, thereby decreasing the solubility of the gas
-The solubility of most solid or liquid solutes increases with increasing temperature

83.

If 5.0 g of marble reacts with 25.0 cm3 hydrochloric acid, which of the following combinations has the fastest reaction rate?

Marble chips and 2.0 mol dm $^{-3}$ HCl $_{(aq)}$

Marble chips and 2.5 mol dm $^{-3}$ HCl $_{(aq)}$

Powdered marble and 2.5 mol dm $^{-3}$ HCl $_{(aq)}$

Powdered marble and 2.0 mol dm $^{-3}$ HCl $_{(aq)}$

View answer & explanation

Correct answer is C

Powdered marble reacts more rapidly with HCl than the chips of marble because:
Surface area of powdered marble is more that of chips of marble and hence there is more collisions between the molecules of reactants
Number of molecules increases.
Energy of activation decreases
Marble chips will not react with HCl

84.

A colourless gas with a pungent smell is evolved when dilute hydrochloric acid is added to a sample of a salt. The gas evolved could turn

acidified K $_{2}$ Cr $_{2}$ O $_{7}$ solution colourless

acidified KMnO $_{4}$ solution colourless

Fe(NO $_{3}$ ) $_{2}$ solution green

Pb(NO $_{3}$ ) $_{2}$ paper black

View answer & explanation

Correct answer is B

If the potassium manganate(VII) solution is acidified with dilute sulfuric acid, the purple solution becomes colourless.

85.

The IUPAC name for CH $_{3}$ CHCH $_{3}$ CHCLCHCH $_{3}$ CH $_{2}$ CH $_{3}$ is

2, 4 - dimethyl - 3 - chlorohexane

3, 5 - dimethyl - 4 - chlorohexane

4 - chloro - 3, 5 - dirnethylhexane

3 - chloro - 2, 4 - dimethylhexane

View answer & explanation

Correct answer is D

- Recognise the functional group in the compound. This will determine the suffix of the name

- Find the longest continuous carbon chain that contains the functional group (it won't always be a straight chain) and count the number of carbon atoms in this chain. This number will determine the prefix (the beginning) of the compound's name

- Number the carbons in the longest carbon chain (Important: If the molecule is not an alkane (i.e. has a functional group) you need to start numbering so that the functional group is on the carbon with the lowest possible number). Start with the carbon at the end closest to the functional group.

- Look for any branched groups:
Name them by counting the number of carbon atoms in the branched group and referring to Table 4.6, these groups will all end in -yl.
Note the position of the group on the main carbon chain. If there is more than one of the same type of branched group then both numbers must be listed (e.g. 2,4 -) and one of the prefixes listed in Table 4.7 must be used. Important: If the molecule is an alkane the branched group must be on the carbon with the lowest possible number.
The branched groups must be listed before the name of the main chain in alphabetical order (ignoring di/tri/tetra).
If there are no branched groups this step can be ignored.

- For the alkyl halides the halogen atom is treated in much the same way as branched groups:
To name them take the name of the halogen atom (e.g. iodine) and replace the -ine with -o (e.g. iodo)
Give the halogen atom a number to show its position on the carbon chain. If there is more than one halogen atom the numbers should be listed and a prefix should be used (e.g. 3,4-diiodo- or 1,2,2-trichloro-). See Table 4.7 for a list of the prefixes.
The halogen atoms must be listed before the name of the main chain in alphabetical order (ignore di/tri/tetra).
If there are no halogen atoms this step can be ignored.
- Combine the elements of the name into a single word in the following order:
branched groups/halogen atoms in alphabetical order (ignoring prefixes)
prefix of main chain
name ending according to the functional group and its position on the longest carbon chain.