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# Chemistry Titration/Moles Multiple Choice Question [PLEASE CHECK] Watch

1. Hi,
This is a multiple choice question that I found and I wanted to check that my working was indeed correct.

When 0.28g of a basic oxide, MO, is reacted with 250cm3 of 0.05moldm–3 hydrochloricacid the excess acid required 50cm3 of 0.05moldm–3 sodium hydroxide solution forneutralisation. Which one of the following is the relative atomic mass of M?
A 12
B 28
C 40
D 56

My working for this question
First, I want to find out how many moles of HCl were excess.
I know that 50cm3 or 0.05dm3 were excess and the concentration of NaOH (Sodium Hydroxide) needed was 0.05moldm-3 so I did the following

0.05 x 0.05 = 0.0025 moles of NaOH were used.

The equation of this reaction is
HCl + NaOH → NaCl + H2O
Where the NaOH and the HCl are in a 1:1 ratio, so the excess of HCl is also 0.025 moles.

I then referred back to the original question.
Since 250cm3 or 0.25dm3 of HCl was used of concentration 0.05moldm-3, I needed to calculate the total number of moles involved in the reaction.

This was moles.
I previously calculated the excess to being 0.0025 moles, so the total used were 0.0125 - 0.0025 moles = 0.01 moles.

I thought that MO was meant to be M2O due to the oxygen's oxidation number.

The equation of the reaction showed the following
M2O + 2HCl → 2MCl + H2O
The M2O and the HCl are in a 1:2 ratio, so the M2O moles used were = 0.005

I also knew of the mass of the M2O to being 0.28, so to calculate the Mr, I did = 56.

Then I subtracted 16 to find the Mr of the Metal to be 40 (Calcium).

Is this correct?

UPDATE - THE ANSWER HAS BEEN FOUND.
2. i think that MO in the question may indicate that M has an oxidation number of +2, like MgO and CaO.
in this case, the equation will be
MO + 2HCl --> MCl2 + H2O
but the mole ratio u used between HCl and MO is correct, C should be right
i might be wrong idk😂

Posted from TSR Mobile
3. (Original post by mega.white)
i think that MO in the question may indicate that M has an oxidation number of +2, like MgO and CaO.
in this case, the equation will be
MO + 2HCl --> MCl2 + H2O
but the mole ratio u used between HCl and MO is correct, C should be right
i might be wrong idk😂

Posted from TSR Mobile

You are correct! I just tried again, and yes your theory seems to work! Thanks very much!

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Updated: April 16, 2016
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