Question

How many atoms are there in 75 grams of calcium?

Answer 1

There are approximately `1.1xx10^24` atoms in `"75 g Ca"`.

Explanation:

First determine mol `"Ca"` by dividing the given grams by its molar mass (atomic weight on periodic table in g/mol). One mole of anything is `6.022xx10^23` anythings, including atoms. Once you have mol `"Ca"`, multiply it by `6.022xx10^23` `"atoms/mol"`.

`(75color(red)cancel(color(black)("g")) "Ca")/(40.078color(red)cancel(color(black)("g"))/"mol")="1.9 mol Ca"`

`1.9color(red)cancel(color(black)("mol Ca"))xx(6.022xx10^23"atoms Ca")/(1color(red)cancel(color(black)("mol Ca")))=1.1xx10^24` `"atoms Ca"` (rounded to two significant figures)