How many moles are 5 grams of calcium ? (Atomic mass of calcium `= 40 u`).

Explanation

We know that:
1 mole of atoms = Gram atomic mass
So, 1 mole of calcium atoms = Grams atomic mass of calcium
`= 40 g`
Now, 40g of calcium = 12 mole of calcium
So, 5g of calcium `= 1/ 40 xx 5` mole
`= 1/8` mole
`= 0.125` mole
Thus, there are `0.125` mole in `5` grams of calcium.
Note, the above problem can also be solbed directly by uing the formula :
`underset("of atom")("Number of moles") = ("Mass of element in grams")/("Grams atomic m ass of element")`
` = 5/40`
`= 1/8`
`= 0.125` mole
Thus, 5 grams calcium constitute `0.125` mole of calcium.
A yet number another way of writing the above formula is by using the term 'molar mass' in place of 'gram atomic mass'. That is :
`underset("of atoms")("Numbr of moles") = ("Mass of element")/("Molar mass of element")`
In the above case, mass of element is 5 grams and molar mass of element is `40 g//"mol"`.