### Higher - Gas Volume Questions

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• #### Message 1.

Posted by MissChemTeacher_Bitesize (U15172316) on Wednesday, 16th January 2013

Hey Guys!

Here are some gas volume questions to get you thinking before your prelims.

1) Silver oxide breaks down when heated as shown by the equation below.

2Ag₂O ----------> 4Ag + O₂

Calculate the mass of silver oxide that would release 200cm³ of oxygen. (Take the molar volume of oxygen to be 23.5 litres per mole.)

2) The gas, hydrogen sulphide can be made by adding iron sulphide to hydrochloric acid as shown in the equation below.

FeS + 2HCl ----------> FeCl₂ + H₂S

100cm³ of 5 mol l¯¹ hydrochloric acid was added to 30g of iron sulphide.

a) Show by calculation that the iron sulphide is in excess.

b) Calculate the volume of hydrogen sulphide gas produced in the reaction. (Take the molar volume of hydrogen sulphide to be 24.0 litres per mole.)

If you need a hand with any questions get in touch and I'll try to help you out.

Good Luck with the revision.

Miss_Chem

• #### Message 2

, in reply to message 1.

Posted by MissChemTeacher_Bitesize (U15172316) on Monday, 21st January 2013

Hello!

Here is a solution to the gas volume problems.

2Ag₂O ----------> 4Ag + O₂

n of moles of O₂ = volume in litres / Molar Volume
n of moles of O₂ = 0.2 / 23.5
n of moles of O₂ = 0.00851

Use the ratio:…. O₂……:…….2Ag₂O.
Mole Ratio:………..1….:……….2
n from above:.0.00851.:…0.017moles

mass of Ag₂O = n of moles x gfm
mass of Ag₂O = 0.017 x 231.8
mass of Ag₂O = 3.94g

• #### Message 3

, in reply to message 2.

Posted by MissChemTeacher_Bitesize (U15172316) on Monday, 21st January 2013

Question 2:

You need to calculate which reactant is in excess before you can complete the molar volume calculation.

FeS + 2HCl ----------> FeCl₂ + H₂S
a)
n of moles of FeS = mass / gfm
n of moles of FeS = 30 / 87.8
n of moles of FeS = 0.342 moles

n of moles of HCl = concentration x volume in litres
n of moles of HCl = 5 x 0.10
n of moles of HCl = 0.5 moles

Mole Ratios: FeS………:………..2HCl
Mole Ratio:……1…………:………….2
Lk @ FeS:…….0.342…….:……..0.684
Lk@ HCl…………0.25…….:……….0.5

As we only have 0.5 moles of HCl we couldn’t possibly ever have 0.684 moles of HCl. We have more FeS than we actually need to the FeS must be in excess.

b) We use the reactant which is not in excess to calculate the number of moles of hydrogen sulphide produced in the reaction.

Mole Ratio: 2HCl……..:……H₂S
Mole Ratio:….2………….:……..1
n moles:……..0.5…………:…….0.25

Volume of H₂S = number of moles x Molar volume
Volume of H₂S = 0.25 x 24.0
Volume of H₂S = 6.0 litres

Hope this helps you out. Cheers Miss_Chem

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