# How many sodium atoms are present in 7Na_3PO_4?

Well, how many sodium atoms in one formula unit of $N {a}_{3} P {O}_{4}$?
I can count $3$ in the one formula unit. In one mole of compound, there are also $3 \cdot m o l$ of sodium.
Now you have quoted $7 \cdot N {a}_{3} P {O}_{3}$, i.e. $7 \cdot \text{formula units}$ OR $7 \cdot \text{moles}$ of sodium phosphate. And thus 3xx7=?? $\text{how many sodium atoms or how many moles of sodium atoms?}$