# Calculate the molarity of the solute in a solution containing 14.2 KCl in 250 mL solution?

## I know molarity is (moles of the solute/ liters of the solution) The issue I dont understand is the 14.2 KCl I dont know what the units are. I am assuming its not grams

Jun 24, 2016

${\text{0.76 mol L}}^{- 1}$

#### Explanation:

I would say that you're dealing with a solution that contains $14.2$ grams of potassium chloride, $\text{KCl}$, in $\text{250 mL}$ of solution.

If this is the case, your strategy here will be to use the molar mass of potassium chloride to calculate how many moles are present in the sample

14.2 color(red)(cancel(color(black)("g"))) * "1 mole KCl"/(74.55color(red)(cancel(color(black)("g")))) = "0.1905 moles KCl"

Now, molarity is simply a measure of a solution's concentration in terms of how many moles of solute it contains per liter of solution.

This means that in order to find a solution's molarity, you essentially must figure out how many moles of solute you have in $\text{1 L}$ of solution.

In your case, you know that $\text{250 mL}$, which is equivalent to $\frac{1}{4} \text{th}$ of a liter, contains $0.905$ moles.

All you have to do now is scale up this solution so that its volume becomes $\text{1 L}$. Simply put, if $\frac{1}{4} \text{th}$ of a liter contains $0.1905$ moles, it follows that $\text{1 L}$ will contain four times as many moles of solute.

1 color(red)(cancel(color(black)("L solution"))) * "0.905 moles KCl"/(1/4color(red)(cancel(color(black)("L solution")))) = "0.76 moles KCl"

So, if $\text{1 L}$ of this solution contains $0.76$ moles of solute, it follows that its molarity is

"molarity" = color(green)(|bar(ul(color(white)(a/a)color(black)("0.76 mol L"^(-1))color(white)(a/a)|)))

The answer is rounded to two sig figs, the number of sig figs you have for the volume of the solution.