# A level chemsitry question

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#1
I'm really not sure how to work this out, any help would be appreciated.
10.0 g of a mixture of copper powder and magnesium powder was mixed with 100cm3 of 1.00 mol dm-3 hydrochloric acid. The copper does not react, but the magnesium does as shown:Mg(s) 2HCL(aq) --> MgCl2(aq) H2(g)The resulting solution was filtered to remove unreacted copper and then made up to 250cm3 with water. 25.0 cm3 of this solution was found to neutralise 36.8cm3 of 0.200 mol dm-3 NaOH. Find the % by mass of the magnesium in the metal powder mixture.
I calculated the answer as 12.15 percent but the answer is 3.2, however I only used the first paragraph of info and I'm not really sure what I'm supposed to do with the second section so that I get the right answer.
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2 months ago
#2

(whatever chemsitry is)
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2 months ago
#3
(Original post by 76891)
I'm really not sure how to work this out, any help would be appreciated.
10.0 g of a mixture of copper powder and magnesium powder was mixed with 100cm3 of 1.00 mol dm-3 hydrochloric acid. The copper does not react, but the magnesium does as shown:Mg(s) 2HCL(aq) --> MgCl2(aq) H2(g)The resulting solution was filtered to remove unreacted copper and then made up to 250cm3 with water. 25.0 cm3 of this solution was found to neutralise 36.8cm3 of 0.200 mol dm-3 NaOH. Find the % by mass of the magnesium in the metal powder mixture.
I calculated the answer as 12.15 percent but the answer is 3.2, however I only used the first paragraph of info and I'm not really sure what I'm supposed to do with the second section so that I get the right answer.
Things you know, or can work out.
n(HCl)total

n(NaOH)
n(HCl) in 25 cm3 that reacted with the NaOH
n(HCl) in 250 cm3

n(HCl) that therefore reacted with the Mg
n(Mg)
m(Mg)
%(Mg)

I got 3.2% too.
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