Question

A solution is prepared by dissolving 3.5 `g` of `KCl` in water & making the volume of solution to 250 `mL`. What is the millimoles of `KCl` dissolved?

Answer 1

Your solution contains 47 mmoles of potassium chloride.

Explanation:

To calculate how many milimoles of potassium chloride you dissolved in solution, all you really need to do is figure out how many moles of the salt you used.

A simple conversion factor will then take you from moles to milimoles.

So, use potassium chloride's molar mass to determine how many moles you added.

`3.5cancel("g") * "1 mole KCl"/(74.55cancel("g")) = "0.0469 moles KCl"`

To convert to milimoles, simply use the fact that one mole contains 1000 milimoles.

`0.0469cancel("moles") * "1000 mmoles"/(1cancel("mole")) = "46.9 mmoles"`

You need to round this off to two sig figs, the number of sig figs you gave for the mass of the salt.

`n_"KCl" = color(green)("47 mmoles")`

You can go on to calculate the molarity of the solution in milimolar, mM.

`C = n/V = "47 mmoles"/(250 * 10^(-3)"L") = 188"mmoles"/"L"`

Once again, round this off to two sig figs to get

`C = color(green)("190 mM")`