[--- (OP)]

how do you work out the number of moles of a substance that has a concentration of 10 mg/L. Do you have to divide 10mg/L by the molar mass?

[CliffordK]

Your molar mass is in grams per mole.

Your target will be in moles / liter if you want a molar concentration.

So...  take:

[ (mg/L) / (1000 mg/g) ] / (g/mol)

And you should end up with mol/L

This, of course, gives you a concentration, not a quantity.  If you want a quantity, you would need to multiply by the number of liters.

Often the milli components will cancel out, milligrams, milliliters, whatever, but you have to be careful with them.

[--- (OP)]

So, for a substance with a concentration of 1mg/L and molar mass of 34.00, would the number of moles be:
[1/1000]/34.00 x 1000= 0.029 moles?

[damocles]

No,

(1) you are a factor of 1000 too high
(2) unless you quote a volume, you cannot answer for "number of mole". The answer you get is "molar concentration" in mole per litre.

A solution of a substance with molar mass 34 g/mol and concentration 1 mg/L corresponds to 0.000029 mole per litre (0.000029 M), or 29 micromole per litre (29 μM). 1 litre of this solution would contain 29 micromole (29 μmol),
but
 if the unexplained "x 1000" at the end of your working is because your sample was 1000 litre, then the number of mole in 1000 litre of this solution is indeed 0.029 mole (0.029 mol or 29 mmol).

[CliffordK]

[1/1000]/34.00 x 1000= 0.029 moles?

or...  you would have a 0.029 millimolar solution.

[CaltechGrad]

What is concentration?  It could be percent by weight, percent by volume, molarity, molality, or something else.

The concentration and molecular mass will not provide a number of moles without more information.  The simplest case would be molarity as the concentration unit:  moles per liter (mol/L).  The molecular mass is grams per mole (g/mol).  You can combine these to eliminate moles but not to find moles.

If you have concentration in mol/L (M), then simply multiply by the volume in liters to get moles.  The molecular mass does not enter into the calculation.

Other concentration units are more involved.  I'll leave them out for now.  Let someone who cares ask about them.

[Bored chemist]

What is concentration?  It could be percent by weight, percent by volume, molarity, molality, or something else.

Other concentration units are more involved.  I'll leave them out for now.  Let someone who cares ask about them.

What is concentration?
That's an easy one.
It's 10 mg/l

I'd like to ask about other units, in particular I'd like to ask what the units you have talked about have to do with the question?

[flr]

how do you work out the number of moles of a substance that has a concentration of 10 mg/L. Do you have to divide 10mg/L by the molar mass?

To compute the number of moles from above problem you would actually need two things: 1) the molar mass and 2) the actual quantity (or volume) of solution.

Knowing the molar mass only, you can covert your concentration from mg/L in mmol/L.
c[mmol/L] = c[mg/l] / M (M=Molar mass).

Then, from the concentration in mmol/L you can easily compute how many moles you have in a certain volume V of solution:
total mili-moles from  given amount of solution = c[mmol/L] * V[L]

If you don't know the total volume of solution, you can only see how many mili-moles you have in 1L solution.