C4

Question 1

Formula for relative atomic mass.

Answer 1

(abundance isotope 1 x mass no. isotope 1) + (abundance isotope 2 x mass no. isotope 2) / total percentage

Question 2

number of atoms in one mole (Avogadro constant)

Answer 2

6.02 x 10^23

Question 3

formula for moles using mass and Ar/Mr

Answer 3

no. moles = mass/Mr

Question 4

formula for mass using concentration and volume

Answer 4

mass = conc. (g/dm^3) x vol. (dm^3)

Question 5

formula for moles using conc. + vol.

Answer 5

moles = conc. (g/dm^3) x vol (dm^3)

Question 6

formula for the percentage yield of a chemical reaction

Answer 6

perc yield = (total mass of product/theoretical mass of product) x 100

Question 7

Describe 4 factors affecting percentage yield.

Answer 7

• reaction may be reversible
• some products may be lost in handling/in equipment
• unexpected and unwanted products may be formed
• reactants may not be pure

Question 8

Formula for perc. atom economy

Answer 8

atom economy = (Mr of desired product / Mr of reactants) x 100

Question 9

Why is atom economy important ?

Answer 9

• conserve natural resources
• reduce pollution
• increase industrial efficiency

Question 10

Formula for number of moles of gas.

Answer 10

no. moles gas = vol. gas (dm^3)/24 (dm^3)

Question 11

How many moles of gas in 36 dm^3 CO2 ?

Answer 11

1.5 moles

Question 12

Name 3 indicators used in titrations and what colours they turn when acid is added.

Answer 12

methyl orange
- yellow to orange

litmus paper
- blue to red

phenolphthalein
- pink to colourless

Question 13

Explain how to carry out a titration.

Answer 13

• wash volumetric pipette with distilled water and some of the alkali being used
• known volume of alkali into conical flask using volumetric pipette and pipette filler
• add a few drops of phenolphthalein into conical flask and swirl to mix
• rinse burette with distilled water, then some of the acid
• fill burette with acid (tap closed) until meniscus line sits at 0 (should be read from eye level)
• with conical flask under the tap, slowly open tap and swirl flask as acid flows in to mix
• close tap when reaction reaches end point (solution changes from pink to colourless
• read what volume of acid was used to neutralise the alkali
• repeat titration until achieve concordant results
• use results to calculate concentration of acid in mol/dm^3

Question 14

In a titration, a 12.5 cm^3 solution of nitric acid reacted exactly with 10 cm^3 of 0.4 mol/dm^3 potassium hydroxide solution.

Calculate the number of moles of potassium hydroxide used.

Answer 14

nitric acid pt. hydrox.
C= 0.32 mol/dm^3 C= 0.4
N= 0.004 mol N= 0.004
V= 0.0125 dm^3 V= 0.01

• convert all cm^3 to dm^3 (divide by 1000)
• use mol = conc. x vol. and rearrange throughout rest of question to get answer

Question 15

Metal X is extracted from an oxide of metal X in a reaction with hydrogen.

The reaction’s equation is:
XO3 + 3H2 -> X + 3H2O

the percentage atom economy for obtaining metal X is 77.3%.

Calculate the Ar of metal X.
(Ar: H = 1, O = 16)

Answer 15

Mr of desired product (X) = x
Mr reactants = X + 54

(x/x+54) x100 = 77.3
100x = 77.3(54+x)
100x = 77.3x + 4174.2
100x-77.3x = 22.7x
22.7x = 4174.2
x = 183.885 (3 d.p)

Question 16

Titanium is extracted from TiO2.

TiO2 + 2Cl2 + 2C -> TiCl4 + 2CO

Calculate the volume of chlorine gas needed to react completely with 100kg of TiO2.
(Ar: O = 16, Ti = 48)
(vol. 1 mol of gas = 24 dm^3)

Answer 16

Mr of TiO2 = 80
100 000g/80g = 1250
2 moles of Cl2 in the equation so:
1250 x 2 = 2500
2500 x 24 = 60 000 dm^3