# Gcse chem maths question help

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#1
Hi, is it possible for someone to check my answers since they don't look correct to me?

In an experiment to find the percentage conversion of magnesium to magnesium oxide, 0.15g magnesium ribbon was heated in air for 5 minutes. The mass of product found to be 0.22g.

Calculate:
a) The number of moles of magnesium at the start of the experiment.

0.15 / 24.3 = 0.00617.... = 0.01
b) The mass of oxygen which reacted with magnesium.
0.22 - 0.15 = 0.07
c) The number of moles of oxygen which reacted.
0.07 / 16 = 4.375 x 10^-3 mol
d) Construct balanced equation for the conversion of magnesium to magnesium oxide and hence the number of moles of magnesium which combined with oxygen.
2Mg + O2 -> 2MgO
(i'm not sure how to do the second part)
e) Calculate the % of magnesium which was converted to magnesium oxide
(0.15 / 0.22) * 100 = 68.18%
f) Describe a simple chemical test which can show not all magnesium had been oxidised.
Last edited by RecallVelocity; 1 year ago
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1 year ago
#2
Your answer for a is correct although it may be simpler to use 24 as the relative atomic mass of magnesium as that is all u willl need at GCSE

B is correct

For c the atomic mass of oxygen is 32 not 16 as it is O2

Your balanced equation is right, for the second part you just look at the big numbers before the elements so in this case 2 moles of magnesium react with 1 mole of oxygen to form 2 moles of magnesium oxide

For e your method is wrong as you need to understand the second part of d and look into which element is the limiting reactant although your answer is quite close to the real answer

Hope this helps if you need any more help I’m happy to try and help you, are you in year 10 or 11? 😊
0
1 year ago
#3
I'd use the equation Mg + 1/2 O2 ---> MgO, as this is the equation for the combustion of magnesium. Otherwise half your answer. Then find the moles of oxygen, and then times by 2 for the moles of magnesium, that reacted, as it's a 0.5 to 1 ratio.

If not all the magnesium had oxidised, then there would be some metal left. You should know that metal + acid ---> salt + hydrogen. Hydrogen gas is tested with a burning splint and makes a "pop" sound if present. (Should be adequate for GCSE).
So I'd say add excess dilute hydrochloric acid, and then test the gas evolving from the test tube with a burning splint.
Last edited by Deggs_14; 1 year ago
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