Pure platinum is too soft to be used in jewelry because it scratches easily. To increase the hardness so that it can be used in jewelry, platinum is combined with other metals to form an alloy. To determine the amount of platinum in an alloy, a 7.804 g sample of an alloy containing platinum and cobalt is reacted with excess nitric acid to form 2.03 g of cobalt (II) nitrate. Calculate the mass percent of platinum in the alloy.

Please help!!

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  1. 7.804 g = mass Co + Pt
    -2.03 g = mass Co
    about 5.8 g = mass Pt but you need a better answer

    %Pt = (mass Pt/mass sample)*100 = ?

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  2. find the moles of cobalt nitrate ... divide 2.03 g by the molar mass (182.9 g)
    ... this is also the moles of cobalt

    find the mass of cobalt ... multiply the moles by the molar mass (58.93 g)

    find % of cobalt in alloy ... divide mass of cobalt by mass of alloy

    mass % of platinum is ... 100% minus % of cobalt

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  3. There is nothing particularly wrong with what you wrote but you don't need all of that work. You already have the mass Co. Your step 1 converts g Co to mols and step 2 converts back to grams so you're back to where you started. Then you find %Co, which the problem doesn't ask for and subtract from 100 to find %Pt. Why not find % Pt directly?

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  4. Hi,
    So I calculated 7.804g - 2.03g which equals to 5.774g. (Is that what you meant by a better answer?) Would it be correct if I used that (5.774g) and divide by the total mass (7.804g) so that it would become ~0.7399? And the answer would be 73.99%? I'm just unsure because in the question, it says cobalt nitrate, not just cobalt. Sorry if this is confusing.

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