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Chemistry moles and concentrations help?

Hi I'm really struggling with a question to do with concentrations and moles based on an experiment we did in class today using 25cm3 of hydrochloric acid and 0.2g of metal X at 0.009mol which I worked out is magnesium

A student carried out an experiment using metal x and hydrochloric acid and collected the hydrogen gas that was produced in the reaction. (I already worked out that the metal is magnesium) They carried out the experiment using 0.12g of the metal, but realised he had only added 20cm3 of 1.00 mol dm3 hydrochloric acid instead of 25cm3. Explain why this mistake would not affect the volume of gas produced.
{Hint: You will need to calculate the moles of HCl and the moles of the metal...)

I worked out that the mol of HCl was 0.020 but I don't know where to go from here.
Any help would be much appreciated!
First you will need to write out your balanced equation, if the metal is magnesium as you stated, the equation is Mg + 2Hcl ---> MgCl2 + H2
Hcl Moles = conc x volume/1000 = 0.02 which you got, now due to the balanced equation Hcl and Mg are the 1:2 ratio so number of moles of Mg is 0.02/2 = 0.01

Now using the equation Number of moles = mass/ Mr
Number of moles x Mr = mass

so = 0.01x24x = mass of Mg = 0.24 g


The mistake doesnt effect gas produced as the Hcl is already in excess
(edited 9 years ago)
Reply 2
Check how much HCl is needed to dissolved the metal sample. Compare that to how much you have.

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