# How many moles of potassium are in 0.256 g?

Mar 16, 2016

$n \approx 0.00655 m o l$

#### Explanation:

Recall that the formula for moles is:

$\textcolor{b l u e}{| \overline{\underline{\textcolor{w h i t e}{\frac{a}{a}} n = \frac{m}{M} \textcolor{w h i t e}{\frac{a}{a}} |}}}$

where:
$\textcolor{m a \ge n t a}{n} =$number of moles
$\textcolor{\mathmr{and} a n \ge}{m} =$mass (grams)
$\textcolor{t e a l}{M} =$molar mass (grams/mole)

Since you already have the mass of potassium, $\textcolor{\mathmr{and} a n \ge}{0.256 g}$, and the molar mass, $\textcolor{t e a l}{39.1 \frac{g}{m o l}}$, which can be determined by looking at a periodic table, the only variable left to solve for is $\textcolor{m a \ge n t a}{n}$, the number of moles. Thus:

$\textcolor{m a \ge n t a}{n} = \frac{\textcolor{\mathmr{and} a n \ge}{m}}{\textcolor{t e a l}{M}}$

$\textcolor{m a \ge n t a}{n} = \frac{\textcolor{\mathmr{and} a n \ge}{0.256 g}}{\textcolor{t e a l}{39.1 \frac{g}{m o l}}}$

$n = 0.256 g \cdot \frac{m o l}{39.1 g}$

$n = 0.256 \textcolor{red}{\cancel{\textcolor{b l a c k}{g}}} \cdot \frac{m o l}{39.1 \textcolor{red}{\cancel{\textcolor{b l a c k}{g}}}}$

$n = 0.006547314578 m o l$

$\textcolor{g r e e n}{| \overline{\underline{\textcolor{w h i t e}{\frac{a}{a}} n \approx 0.00655 m o l \textcolor{w h i t e}{\frac{a}{a}} |}}} \Rightarrow$rounded off to $3$ significant figures