how many moles are in 9.8 grams of calcium?

when the number of moles Ca = mass of Ca / molar mass of Ca.

and we can get the molar mass of Ca, it is = 40 g/mol

and we have already the mass of Ca (given) = 9.8 g

so, by substitution: the moles Ca = 9.8 g / 40 g/mol

= 0.245 moles