Molar Volume Calculations

Question 1

what is a molar volume

Answer 1

a volume of gas that contains one mole of that gas

Question 2

how do the molar volumes vary for all gases for given temperatures and pressures

Answer 2

they are approximately the same

Question 3

what are the most often used values for the given temperature and pressure and what is it abbreviated to

Answer 3

• the temperature is room temp at 298K
• while the pressure is at the standard pressure of 1atm (101325 kPa)
• the abbreviation is RTP (room temperature pressure)

Question 4

what is the molar volume for all gases at RTP and what is its symbol

Answer 4

• 24dm3 or 24000cm3

- Vm = 24dm3mol-1

Question 5

using the units of Vm, what would the equation for molar volume be

Answer 5

molar volume = volume of gas (dm or cm) / amount in moles

Question 6

what is the amount in moles of CO in 3.8dm3 of cabon monoxide

Answer 6

• one mole of CO at RTP would be 24cm3
• so to work out how many moles you have rearrange the equation
• giving moles = volume of gas / molar volume
• 3.8 / 24 = 0.158 mols

Question 7

what is the volume of 0.356mol of hydrogen gas

Answer 7

• rearrange to volume of gas = moles x molar volume

- 24 x 0.356 = 8.54dm3

Question 8

a piece of Mg with mass 1g is added to an excess of dilute hydrochloric acid. What volume of hydrogen gas is formed? The equation is Mg + 2HCl = MgCl2 + H2

Answer 8

• you need to work out the moles of Mg there are
• you can then use this to compare the ratio / coefficients in the equation for how many moles of hydrogen gas there would be
• as one equation mole of Mg produces one e-mole of hydrogen gas, the moles of Mg used would be equal to hydrogens
• moles = mass / Mr, so 1 / 24/3 = 0.0412 mols
• volume of gas = moles x molar volume, so 0.0412 x 24 = 0.988dm3

Question 9

in the reaction CaCO3 + 2NHO3 = Ca(NO3)2 + H2O + CO2, 100cm3 of carbon dioxide is formed. What mass of calcium carbonate is needed for this?

Answer 9

• work out the moles of carbon dioxide produced
• the ratio of the coefficients between calcium carbonate and carbon dioxide is 1:1 so you need the same number of moles of CaCO3
• moles = volume / molar mass so 100 / 24000 = 0.004167
• mass = moles x Mr, so 100 x 0.004167 = 0.417g

Question 10

in the reaction (NH4)2SO4 + 2NaOH = Na2SO4 + 2H2O + 2NH3, what volume of ammonia is formed by reacting 2.16g of ammonium sulfate with excess sodium hydroxide

Answer 10

• the same steps as the others, but this time the ratio coefficients are 1:2
• Mr of (NH4)2SO4 = 132gmol-1
• moles = mass / Mr so 2.16 / 132 = 0.0164 mols
• as one e-mole of that makes 2 e-moles of ammonia, you times it by 2 to get 0.0328
• then you do volume = moles x molar volume to get 0.0328 x 24 = 0.785dm3