Given that the molecular weight of KC1 is 74.55 g, how many grams would you need to make 300 ml of a 200 mM solutio?
The molarity of a solution is equal to the number of moles of the specified substance that are dissolved in one liter of solution (1000 ml of solution). Therefore we have
in 1000 ml solution ...200 mM
in 300 ml solution......x mM
x =300*200/1000 =60 mM
Because the molecular weight of KCl is 74.55, it means the mass of one mole of KCl is 74.55 grams. Thus
1 Mole of KCl has .........74.55 grams
60 mMoles of KCl have...y grams
y = (74.55*60*10^-3)/1 =4.473 grams KCl
To make 300 ml of a 200 mM solution of KCl one needs 4.473 grams KCl.