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 7.518 g sample of an alloy containing platinum and cobalt is reacted with excess nitric acid to form 1.23 g of cobalt(ii) nitrate. calculate the mass percent of platinum in the alloy.

94.7%

Explanation:

The equation of the reaction:

Co(s) + 2HNO3(aq) > Co(NO3)2(aq) + H2(g)

Number of moles = mass of substance/ molar mass of cobalt II nitrate

Number of moles= 1.23g/182.943 g/mol = 6.7×10^-3 moles

From the reaction equation, 1 mole of Cobalt metal gave 1 mole of Cobalt II nitrate

x moles of Cobalt will give 6.7×10^-3 moles of Cobalt II nitrate

x= 6.7×10^-3 moles of Cobalt metal

Mass of cobalt metal = number of moles × molar mass

Molar mass of cobalt metal= 59gmol-1

Mass of cobalt= 6.7×10^-3 moles× 59gmol-1

Mass of cobalt= 0.40 g

Mass of platinum in the sample= 7.518-0.40= 7.118 g

% mass of platinum= 7.118/7.518 × 100

% mass of platinum= 94.7%

take a better picture and ill do it

i believe you are right for all except the 3rd ( look at sqdancefan his answer is much better)