Question

A solution with a volume of 0.25 liters contains 20 grams of hydrogen fluoride, HF. What is the molarity of the solution?

Answer 1

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

Explanation:

In order to find a solution's molarity, you must determine how many moles of solute you get per liter of solution.

To determine the number of moles of hydrofluoric acid present in your sample, use the compound's molar mass. Hydrofluoric acid has a molar mass of `"20.01 g mol"^(-1)`, which means that one mole of this compound has a mass of `"20.01 g"`.

Since your sample has a mass of `"20 g"`, you can say that it contains

`20 color(red)(cancel(color(black)("g"))) * "1 mole HF"/(20.01color(red)(cancel(color(black)("g")))) = "0.9995 moles HF"`

Now, you know that this many moles are being dissolved in `"0.25 L"`, so you can say that `"1 L"` of this solution will contain

`1 color(red)(cancel(color(black)("L solution"))) * "0.9995 moles HF"/(0.25color(red)(cancel(color(black)("L solution")))) = "3.998 moles HF"`

Rounded to one significant figure, the molarity of the solution will be

`color(green)(bar(ul(|color(white)(a/a)color(black)("molarity HF solution" = "4 mol L"^(-1))color(white)(a/a)|)))`