☰ Contents · Chemistry (General chemistry)

Molar concentration

Lessons 18 · 1 lessons · S. Masharipov, A. Mutalibov, E. Murodov, H. Islomova. General chemistry, Grade 11, 1st edition. G‘afur G‘ulom Publishing and Printing House, Tashkent, 2018
18

Molar concentration

Textbook: pp. 85–88
GoalExplain molar concentration (C_M = n/V) and use it to solve problems on the amount and mass of solute, the volume of a solution and dilution.
New words
molar concentration · molyar konsentratsiyamol/L (M) · mol/l (M)volumetric flask · o‘lchov kolbasidilution · suyultirish
Explanation

The number of moles of solute in 1 L of solution is the molar concentration of the solution: C_M = n/V, where n is the amount of solute (mol) and V the volume of solution (L). The unit is mol/L or M («molar»); for example 0.1 M HCl has 0.1 mol of HCl in 1 L. From the formula n = C_M·V and V = n/C_M; to bring in mass use n = m/M: m = C_M·V·M. If the volume is given in mL, convert it to litres (1 L = 1000 mL). On diluting a solution with water the amount of solute does not change, so C₁·V₁ = C₂·V₂. To prepare a solution of exact molar concentration the substance is weighed and made up with water to the mark in a volumetric flask; acids and alkalis are handled only under the teacher’s supervision, with goggles and gloves.

Worked examples
1.2 mol of KNO₃ is dissolved to make 400 mL of solution. V = 0.4 L; C_M = 1.2/0.4 = 3 mol/L.
To prepare 500 mL of 0.2 M NaOH the mass of NaOH is: n = 0.2·0.5 = 0.1 mol; m = 0.1·40 = 4 g.
Class activity

Calculation task: each group calculates the mass needed to prepare 250 mL of a 0.1 M solution of a different substance (the actual preparation is carried out only by the teacher).

Practice
1
49 g of H₂SO₄ (M = 98 g/mol) is dissolved to make 500 mL of solution. Find the molar concentration (mol/L).
2
In how many litres of a 0.5 M solution are there 2 mol of solute?
3
How many grams of NaOH are in 250 mL of 0.8 M NaOH solution (M = 40 g/mol)?
4
100 mL of a 6 M solution is diluted with water to 600 mL. What is the new concentration in mol/L?