Question

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

Answer 1

`"0.76 mol L"^(-1)`

Explanation:

I would say that you're dealing with a solution that contains `14.2` grams of potassium chloride, `"KCl"`, in `"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 `"1 L"` of solution.

In your case, you know that `"250 mL"`, which is equivalent to `1/4"th"` of a liter, contains `0.905` moles.

All you have to do now is scale up this solution so that its volume becomes `"1 L"`. Simply put, if `1/4"th"` of a liter contains `0.1905` moles, it follows that `"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 `"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.