Calculating Reacting Masses
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Assessment
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Mr Boyle
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Chemistry
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5th Grade
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21 plays
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Hard
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15 questions
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1.
Fill in the Blank
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
2.
Fill in the Blank
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
3.
Fill in the Blank
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 Mr of K2O:
(39x2) + (16x1) = 94 g/mol
5) Calculate mass of K2O:
m = nM = 0.1 x 94 = 9.4 g
4.
Fill in the Blank
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 Mr 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
5.
Fill in the Blank
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
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