(c)
2.1g / (126.043 g/mol) = 0.0166609808 moles = 1.00334773 × 10^22 particles

1.00334773 * 10^22 ions

(d)
0.36 g / (24.305 g/mol) = 0.01481176712 moles = 8.91985479 × 10^21 particles

8.91985479 × 10^21 atoms

(e)
8 grams of calcium / ( 40 grams / mole of calcium ) = 0.2 moles of calcium

0.2 moles of magnesium * (24 grams / mole of magnesium) = 4.8 grams of magnesium

(f)
0.2 grams of magnesium / (24 grams / mole of magnesium) = 0.008333 moles

(g)
7 grams of iron / ( 56 grams / mole of iron ) = 0.125 moles of iron

0.125 moles of magnesium * (24 grams / mole of magnesium) = 3 grams

(h)
6.3 grams / ( 63 grams / mole of nitric acid) = 0.1 moles of nitric acid

0.1 moles of nitric acid * 3 oxygen atoms per nitric acid molecule = 0.3 moles of oxygen atoms

(i)
6.4 grams / ( 16 grams / mole of methane ) = 0.4 moles of methane

4 hydrogen atoms per methane molecule * 0.4 moles of methane = 1.6 moles of hydrogen atoms