# What molar quantity is represented by a 5.75*g mass of sodium?

Sep 13, 2017

For all these problems we take the quotient.....

#### Explanation:

$\text{Number of moles}$ $=$ $\text{Mass of stuff"/"Molar mass of stuff}$

Where $1 \cdot m o l \equiv 6.022 \times {10}^{23} \cdot \text{individual items of stuff}$.

$\text{Moles of sodium}$ $=$ $\frac{5.75 \cdot g}{22.99 \cdot g \cdot m o {l}^{-} 1} = 0.250 \cdot m o l$.

And note the dimensional consistency of the answer....i.e.

$\frac{5.75 \cdot \cancel{g}}{22.99 \cdot \cancel{g} \cdot m o {l}^{-} 1} = 0.250 \cdot \frac{1}{\frac{1}{m o l}} = 0.250 \cdot m o l$.

How did I know the molar mass of sodium? How will you know it if you are asked in an exam?

And to get the molar mass of sodium, chlorine, and copper, I look at that thing that tells me the molar masses of these elements. I forget its name....and I get molar masses of $22.99 \cdot g \cdot m o {l}^{-} 1$, $N a$; $35.45 \cdot g \cdot m o {l}^{-} 1$, $C l$; $63.55 \cdot g \cdot m o {l}^{-} 1$, $C u$.

Note that chlorine is commonly encountered as the diatomic molecule, i.e. $C {l}_{2}$, but given the terms of the question, I feel I am justified in specifying a mole of chlorine ATOMS......But what would be the mass of $6.022 \times {10}^{23} \cdot \text{chlorine molecules}$?