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VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Chemistry => Topic started by: yabbaboo on December 15, 2011, 11:09:21 am
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Hi,
Can anyone please help me? The question is:
A sliver coin was analysed to determine its silver content. Shavings from the coin were dissolved in concentrated nitric acid to oxidise the silver atoms to silver ions. An excess of sodium chloride solution was added to precipitate the sliver ions. 1.30g of coin shavings produced 1.31 grams of dried precipitate. Calculate the percentage by mass of sliver in the coin.
I am not really sure how to tackle this problem :(
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Hint: start by finding the reaction equations to see what is actually happening to the silver.
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1) HNO3 + Ag --> AgNO3 + H2
2) AgNO3 + NaCl --> AgCl + NaNO3
However, I don't believe that the first equation is a possible reaction...
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analysing the reactions taking place will aid your understanding of redox and precipitate chemistry but the reactions is not essential.
the essential part is linking the mass of precipitate to the mass of silver in the coin. silver chloride is AgCl
there are two ways to find the mass of an element in a compound when the compound's formula and mass are known.
1) find amount in mole of compound and use subscript to find amount in mole of element, then use molar mass of element to find mass of element. eg. in this example, n(Ag)= n(AgCl) and m(Ag)= n(Ag).M(Ag)
2) use percentage composition shortcut (recommended for speed but more error- prone): m(Ag)= [M(Ag)/M(AgCl)].m(AgCl). i.e the fraction is the proportion of AgCl that is Ag by mass.
once you have found m(Ag), find the percentage composition using the relevant formula.