How many carbon atoms are there in a diamond (pure carbon) with a mass of 47 mg ?

2.4 x 10^21 First, calculate the number of moles of carbon you have by dividing the mass (in grams) by the atomic weight of carbon. Atomic weight of Carbon = 12.0107 0.047 g / 12.0107 g/mol = 0.003913 mole Now multiply by avogadro's number 0.003913 * 6.0221409 x 10^23 = 2.35646 x 10^21 Since you only have 2 significant figures, round the result to 2 significant figures 2.4 x 10^21

And a) oxygen i believe