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?

1 Answer
Jul 13, 2015

Your solution contains 47 mmoles of potassium chloride.


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")#