# Question #b148b

May 21, 2016

$5.23 \times {N}_{A} \text{ water molecules}$, where ${N}_{A}$ $=$ $\text{Avogadro's number}$.

#### Explanation:

So two questions.

(i) What is ${N}_{A}$, and,

(ii) Why should we use such an absurdly large number?

Avogadro's number, ${N}_{A}$ $=$ $6.022 \times {10}^{23}$.

So was this number plucked out of the air? In fact, it turns out that ${N}_{A}$ $\text{^1H" atoms}$ have mass of $1 \cdot g$ precisely, and ${N}_{A}$ $\text{^16O" atoms}$ have mass of $16 \cdot g$ precisely.

Avogadro's number is thus the link between grams and kilograms, that which we can weigh and measure in the lab, and the micro world of atoms and molecules, that of which we can theorize and conceive.

If ${N}_{A}$ water molecules have a mass of $18.02 \cdot g$, what is the mass of your quantity?