How to calculate molal concentration

How to calculate molal concentration

Molar concentration – the size which shows how many moths of any substance contain in one liter of solution. That is it is one of types of indicators of concentration. Often there is a task: to count molar concentration of any given solution.


1. Let's assume, you have the solution of 300 milliliters containing 18 grams of sodium saltpeter (that is nitrate of sodium or sodium nitrate). It is necessary to calculate its molar concentration.

2. Remember for a start that a formula of this substance – NaNO3. And also that in number molar mass of any substance is equal to its molecular weight, differing only in dimension. Count the molecular mass of nitrate of sodium: 23 + 14 + 16*3 = 85 grams/mol.

3. Therefore, if 85 grams of nitrate of sodium contained in 1 liter of solution, it would be one-molar (1M) solution of this substance. But at you not 85, but 18 grams, and volume of solution not of 1000 milliliters, but only 300. Make simple calculation: 18*1000/(85*300). You receive the answer: 0.70588 M. Ili, in round figures, 0.706 M. Vot such is molar concentration of the available sodium nitrate solution. Certainly, if you do not need high precision, it is possible to take concentration even for 0.7M.

4. Well, and if statements of the problem are modified? Here, for example, there are 500 milliliters of 20% solution well-known to you substances – table salt (it – chloride sodium). Also it is required to count its molar concentration. How to make it?

5. Here too there is nothing difficult at all. First of all, remember definition of percentage concentration. It is the mass fraction showing how many substance contains in the lump of solution or fusion, or mix of substances. That is for a start you should establish the mass of the available amount of solution. Having glanced in the table of density, you will see: ρ20% NaCl solution at the room temperature it is equal 1.1478 grams/ml. That is the weight of 500 ml of 20% solution will be: 500*1.1478 = 573.9 grams. Or, in round figures, 574 grams.

6. And everything becomes farther already as easy as shelling pears. 20% of 574 grams make: 0.2*574 = 114.8 grams – here so much salt contain in 500 ml of solution. Respectively, 1 liter of such solution would contain 229.6 grams of sodium chloride. If it was 1M solution, then 1 liter would contain 58.5 grams of salt. That is concentration of your solution makes: 229.6/58.5 = 3.92 M. Zadach it is solved.

Author: «MirrorInfo» Dream Team