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?

Md Ariful Islam

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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 , 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 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 to get

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

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