Calculating Reacting Masses

What mass of hydrogen is produced when 192 grams of magnesium reacts with excess hydrochloric acid?

Answer explanation

1) Write a balanced equation for the reaction:

Mg + 2HCl --> MgCl₂ + H₂

2) Calculate the number of moles of Mg:

n = m/M = 192/24 = 8 moles

3) Find molar ratio from equation:

1 (Mg) : 1 (H₂), so 8:8

4) Calculate mass of hydrogen produced:

m = nM = 8 x 2 (H₂ because hydrogen is diatomic) = 16 g

What mass of oxygen is needed to react with 8.5 grams of hydrogen sulfide? The products of this reaction are sulfur dioxide and water.

Answer explanation

1) Write a balanced chemical equation for the reaction:

2H₂S + 3O₂ --> 2SO₂ + 2H₂O

2) Calculate molar mass of hydrogen sulfide:

(1x32) + (2x1) = 34 g/mol

3) Calculate moles of hydrogen sulfide:

n=m/M = 8.5/34 = 0.25 moles

4) Work out molar ratio:

2:3 ratio, so 0.25 : 0.375 (divide 0.25 by 2 then multiply by 3). 0.375 moles of O₂

5) Calculate the mass of oxygen:

m=nM = 0.375 x 32 = 12 g

What mass of potassium oxide is formed when 7.8 g of potassium is burned in oxygen?

Answer explanation

1) Write a balanced equation for the reaction:

4K + O₂ --> 2K₂O

2) Calculate moles of K:

n=m/M = 7.8/39 = 0.2 moles

3) Work out moles of potassium oxide from molar ratio:

4:2, so 2:1 ratio. 0.2 moles of K produces 0.1 moles of K₂O

4) Calculate M_r of K₂O:

(39x2) + (16x1) = 94 g/mol

5) Calculate mass of K₂O:

m = nM = 0.1 x 94 = 9.4 g

Railway lines are welded together by the Thermite reaction, which produces molten iron.  What mass of iron is formed from 1 kg of iron oxide?

 

                                Fe₂O₃ + Al  --> Fe  +  Al₂O₃

Answer explanation

1) First step is to balance the equation given in the question:

Fe₂O₃ + 2Al  --> 2Fe  +  Al₂O₃

2) Calculate M_r of Fe₂O₃:

(56x2) + (16x3) = 160 g/mol

3) Calculate moles of iron oxide:

n = m/M = 1000/160 = 6.25 moles

4) Use molar ratio to determine moles of iron produce:

1:2 ratio, so 6.25 : 12.5 moles of Fe

5) Calculate mass of Fe:

m = nM = 12.5 x 56 = 700 g

What mass of oxygen is required to oxidise 10 grams of ammonia to NO?

Answer explanation

1) Write a balanced equation:

4NH₃ + 5O₂ --> 4NO + 6H₂O

2) Calculate moles of ammonia:

n = m/M =10/17 = 0.588 moles

3) Work out moles of oxygen using molar ratio:

4:5 ratio, so 0.588 : 0.735 [Divide by 4 and multiply by 5]

4) Calculate mass of oxygen:

m = nM = 0.735 x 32 = 23.52 grams

What mass of aluminium oxide is produced when 135 g of aluminium is burned in oxygen?

Answer explanation

1) Write a balanced equation:

2Al + 3O₂ --> Al₂O₃

2) Calculate moles of Al:

n = m/M, = 135/27 = 5 moles

3) Calculate moles of Al₂O₃ from the molar ratio:

2:1 ratio, so 5 : 2.5 moles of aluminium oxide

4) Calculate M_r of Al₂O₃:

(2x27) + (3x16) = 102 g/mol

5) Calculate mass of Al₂O₃:

m=nM = 2.5 x 102 = 255 g

What mass of iodine is produced when 7.1 grams of chlorine reacts with excess potassium iodide?

Answer explanation

1) Write a balanced equation:

Cl₂ + 2KI --> 2KCl + I₂

2) Calculate moles of Cl₂:

n=m/M = 7.1/71 = 0.1 moles

3) Use molar ratio to calculate moles of I₂:

1:1 ratio, so 0.1 : 0.1

4) Calculate mass of iodine:

m = nM = 0.1 x 254 = 25.4 g

What mass of hydrogen is needed to react with 32 grams of copper (ii) oxide?

Answer explanation

1) Write a balanced equation:

CuO + H₂ --> Cu + H₂O

2) Calculate moles of CuO:

n=m/M = 32/79.5 = 0.4 moles

3) Use molar ratio to calculate moles of H₂:

1:1 ratio, so 0.4:0.4

4) Calculate mass of hydrogen:

m = nM = 0.4 x 2 = 0.8 g

What mass of oxygen is formed when 735 grams of KClO₃ decomposes to produce oxygen and a salt. Give your answer to 3 s.f.

Answer explanation

1) Write a balanced chemical equation:

2KClO₃ --> 2KCl + 3O₂

2) Calculate moles of KClO₃:

n=m/M = 735/122 = 6.02 moles

3) Calculate moles of oxygen using molar ratio:

2:3 ratio so 6.02:9.03 moles O₂

4) Calculate mass of oxygen:

m=nM = 9.03 x 32 = 288.96 g

5) Round answer to 3 s.f.:

288.96 -> 289 g

What volume of hydrogen is produced when 195 mg of potassium is added to water? Give your answer in cm³.

Answer explanation

1) Write a balanced equation for this reaction:

2K + 2H₂O --> 2KOH + H₂

2) Calculate moles of K:

n=m/M = 0.195/39 = 0.005 moles

3) Calculate moles of hydrogen using molar ratio:

2:1, so 0.0025 moles of H₂

4) Calculate volume of hydrogen by multiplying by the molar gas volume of 24 L mol^-1:

24 x 0.0025 = 0.06 L

5) Convert L to cm³ by multiplying by 1000:

0.06 x 1000 = 60 cm³