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Chemical Analysis Question

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ice_blockie:
I was doing some chem work and I came across this question. .I have no idea (well I did, but I couldn't work it out)...anyway can anyone do this?

5.00 g of a mixture of magnesium and zinc was allowed to completely react with dilute sulphuric acid as described by the chemical equation, where M is represents either magnesium or zinc:

M(s) + H2SO4 (aq) ----> MSO4 (aq) + H2 (g)

If 0.250g of hydrogen gas was produced in the reaction, what would the percentage composition of magnesium in the mixture be?

Thanks!

Collin Li:


, but also: ,
where and are the molar proportions of magnesium and zinc respectively.



Substitute this into the equation derived from above:









Therefore the molar proportion of magnesium is 61.8%. We need a mass composition, however:

Since we know there is 0.125 mol of the metal, then we also know:

Therefore, (w/w)

midas_touch:
Coblin fails, the answer is 37.6%.

Collin Li:

--- Quote from: midas_touch on January 28, 2008, 07:58:20 pm ---Coblin fails, the answer is 37.6%.

--- End quote ---

I didn't round off early like you :P. The answer was 37.54, so it is not 37.6.

Mao:
side-question:

dealing with sig figs, do you round at each step or round last??

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