Molarity is moles of solute per litre of solution. Water expands as the temperature increases, so the volume of the solution also increases. You have the same number of moles in more litres, so the molarity is less at higher temperatures.
Assume that you have a solution that contains 0.2500 mol of NaOH in 1.000 L of solution (0.2500 M NaOH) at 10 °C. At 30 °C, the volume of the solution is 1.005 L, so the molarity at 30 °C is
#(0.2500 mol)/(1.005 L)# = 0.2488 M
This may not seem like a big difference, but it is important when you need more than two significant figures in a calculation.
MORAL: If you use molarities in your calculations, make sure that they are all measured at the same temperature.
C is the concentration
n is the number of moles and
V is the volume of solution
So firstly you need to know what is in the solution, I'll take HCl as an example:
Say you have 5g of HCl in a 500ml solution. You must first find the number of moles which is the mass in grams divide by the molar mass of HCl which would be 36.45g/mole. (1g/mole is Hydrogen and 35.45g/mole is Cl)
You get that answer then you divide by your volume in litres, which is 0.5L.
Your answer's unit will mol/L which is sometimes also known as just M.
Should get an answer of 0.27 mol/L for this example :)