Calculations using Reacting Masses

Question 1

for the reaction SO3 + H20 = H2S04, what mass of sulfur troxide is needed to form 75g of sulfuric acid

Answer 1

• Mr of H2SO4 = 98gmol-1
• moles = mass / mr
• 75 / 98 = 0.765 moles
• Mr of SO3 = 80
• mass = moles x Mr
• due to 1:1 rato
• 80 x 0.765 = 61.2g

Question 2

for the reaction 2NH3 + H2SO4 = (NH4)2SO4, what mass of ammonia is needed to form 100g of ammonium sulfate

Answer 2

• Mr of (NH4)2SO4 = 132gmol-1
• moles = mass / Mr
• 100 / 132 = 0.758 moles
• Mr of 2NH3 = 34gmol-1
• mass = moles x Mr
• 34 x 0.758 = 25.7g

Question 3

what is an alternate way of doing that calculation when you take the moles from the equation into account

Answer 3

• 100/ 132 = 0.758 moles
• Mr of NH3 = 17gmol-1
• ratio of moles between 2NH3 and (NH4)2SO4 = 2 : 1
• so, 0.758 x 2 = 1.51 moles of NH3
• mass = moles x Mr
• 1.51 x 17 = 25.7g

Question 4

why could you need to use the reacting masses of substances to identify the equation rather than using the equation to work out the reacting masses

Answer 4

• because there are sometimes two or more possible reactions for the same reactants
• so the reacting masses can be used to work out which one has occured

Question 5

what is a hydrate

Answer 5

a compound containing water of crystallisation

Question 6

what can happen when you heat a hydrate to decompose it

Answer 6

it can decompose into another hydrate and an anhydrous compound

Question 7

what is an anhydrous compound

Answer 7

a pure compound

Question 8

16.7g of a hydrate of sodium carbonate Na2CO3.10H2O is heated at a constant temperature for a specified time until the reaction is complete. a mass of 3.15g of water is obtained. what is the equation for the reaction occurring?

Answer 8

• Mr of Na2CO3.10H20 = 286gmol-1
• Mr of H20 = 18gmol-1
• moles = mass / Mr
• 16.7 / 286 = 0.0584 moles of sodium carbonate
• 3.15 / 18 = 0.175 moles of water
• ratio of between them is 1 : 3, so 3 moles of water for 1 mole of sodium carbonate
• so, Na2CO3.10H20 = Na2CO3.7H20 + 3H20

Question 9

what is the most important step when working out what the formula is

Answer 9

working out the ratio between the reactant and one of the products by calculating their moles

Question 10

What is an alternate and more algebraic method to working the equation out

Answer 10

• Mols of Na2CO3. 10H2O = 0.0584
• Mr of xH2O formed = 18x
• Given that 3.15g of 0.0548 mols of water formed, so mass / mols = Mr
• 3.15 / 0.0584 = 54
• 18x = 54 so x = 3
• giving 3H2O and the rest of the equation