[Bronx20]

Hey guys, nice site you have here.

Im stuck on one question and would like some pointers if possible.

The question is, "If you had 1.5 g of NaCl, and you wanted to make a NaCl solution with a concentration of 20 mM, what volume would you have to make the solution up to (NaCl - 58.5 amu)?"

My main problem is getting my head around '20 mM'.

Am I right in assuming that the NaCl is 58.5 g/mol?

Would I then divide the 20 mM by this?

[oldddog]

OK, so I always look at these types of questions by breaking the infomation down to "first principles".
1. m(NaCl) = 1.5g
2. Mw(NaCl) = 58.5g/mol
3. c(NaCl)req = 20mM = 20x10^-3mol/L
and
n(NaCl) = m/Mw = 1.5g/58.5g/mol =
since c(NaCl) = n(NaCl)/V(NaCl) we can rearrange the equation to find the volume,V.
V(NaCl) = (1.5g/58.5g/mol)/(20x10^-3mol/L) = 1.282L or 1282mL
I hope I got this right and have helped.

[macdonda]

Oldddog is right - I used to look at these problems in a similar way:

Remember, you're starting with a fixed number of moles of NaCl.  You're just adding water so the number of moles NaCl in the solution won't change.  How do you get the # moles; well, just look at the units: molecular weight is in g/mol and mass is in g - if you divide g/(g/mol), you're left with the unit mol.  So, to get the # mol, take mass/molecular weight.  

Say the # of mol is X, you know that you start with X and finish with X (See above); once again, look at the units of concentration (mol/L) and volume (L); if you multiply the two together (mol/L)*(L) = mol

So, you can set up the equation,

             X = (mol/L)*(L) = mol

In other words, the number of moles you start with is the number of moles you finish with

We know that mol/L is concentration (C) and L is volume (V) thus

             X = C*V

You know the #mol X and C = is the final concentration you want so to get the volume from above,

             V = (X/C)

When doing calculations I make sure to keep a close eye on units – they’ll tell me if I made a mistake

[Bronx20]

Excellent, thanks a lot for the replies.

I have always has problems with these questions as I cant work backwards very well.