I have to make 100mL of 400mM NaOH. How do I go about calculating what I need to prepare this?
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This is relatively easy. First: Look up the molecular weight of sodium hydroxide: Wikipedia gives a molar weight of 39.997 grams/mole (lets round this to 40 g/mole thats easier to calculate and the error in this is marginal).
So when you dissolve 40 g of it in 1 l of water, you have a 1000 mM (or 1 M) solution. Since you only want to have a 400 mM solution, you need to multiply the final concentration with the molar weight to get the amount of NaOH for one litre: 40 g/mole * 0.4 mole/l = 16 g of NaOH per litre. Since you only need 100 ml, you only need 1/10th of the NaOH (1.6 g).
So to prepare 100 ml of a 400 mM NaOH solution you dissolve 1.6 g of NaOH in 80ml of water and fill up to 100ml in a appropriate messuring cylinder.
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$\begingroup$ Thank you. I have struggled with these types of problems since we were first introduced to them. For future reference, I would do a similar technique if say I needed a different amount of solute? $\endgroup$ Commented Sep 17, 2014 at 15:36
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$\begingroup$ This is usually the easiest way. Calculate the amount for 1l (since this is usually the basis) and the take the fraction for the actually needed amount. With the calculation above you could easily calculate this for 250ml or also 600ml. $\endgroup$– Chris ♦Commented Sep 17, 2014 at 15:38