Question

what is the mass of 1.20x10^25 atoms of Sodium?

Answer 1

For answer this question you need to use Avogadro constant. In chemistry and physics, the Avogadro constant is defined as the number of constituent particles (usually atoms or molecules) in one mole of a given substance. It has dimensions of reciprocal mol and its value is equal to $6.02 \times 10^{23} \mathrm{~mol}^{-1}$. Atomic weight of Na is 22.99, it means that $6.02 \times 10^{23}$ particles weights $22.99 \mathrm{~g}$. Using this:

22.99g --- 6.02 * 10²³

X g --- 1.20*10²⁵

X = 458.27 g