Question

If we have a pure $MgC{O}_3$ sample that contains a total of $1.26\text{millimoles}$ of electrons and if the number of moles of $MgC{O}_3$ present in the sample is $x\times {10}^{-5}$, then the value of $x$ is:

Answer 1

One molecule of $MgC{O}_3$ contains $=12+6+(8\times 3)=42$ electrons.
So, number of electrons in $1\text{mole}$ of $MgC{O}_3=42\times 6.023\times {10}^{23}$
Therefore, $1.26\text{millimoles}$ of electrons will present in
$=\frac{1.26\times {10}^{-3}\times 6.022\times {10}^{23}}{42\times 6.023\times {10}^{23}}\text{moles}=3\times {10}^{-5}$ moles of $MgC{O}_3$