Concentration

Amount Concentration (c) is the amount (in moles) of solute dissolved per litre of solution.  The symbol for this is mol/L or M (pronounced molar).

Amount concentration = amount of solute (mol) / volume of solution (L)

C = n/V

A Stock Solution is a concentrated solution that is used to prepare dilute solutions for actual use

Sample Problem #1

Sulfuric acid is a solution of hydrogen sulfate, H2SO4, in water.  When concentrated, it is extremely corrosive.  A 25.0 mL sample of concentrated sulfuric acid contains 44 g of hydrogen sulfate. Calculate the amount concentration of the solution

n = m / M

n = 44 g / 98.08 g/mol

n = 0.4486 mol

m  = 44 g

M = 98.08 g/mol

n = ?

V = 25 mL  or 0.025 L

c  = ?

c  = n / V

c  = 0.4486 mol / 0.025 L

c  = 18 mol / L

Sample Problem #2

Household bleach is a 0.67 mol/L solution of sodium hypochlorite, NaClO. What mass of sodium hypochlorite is required to prepare 225 mL of bleach?

nNaClO = c x  V

nNaClO =  0.67 mol / L x 0.2250 L

nNaClO = 0.01508 mol

cNaClO = 0.067 mol/L

VNaClO = 225.0 mL or 0.2250 L

nNaClO = ?

MNaClO= 74.44 g/mol

mNaClO = ?

mNaClO = n x M

mNaClO = 0.01508 mol x 74.44 g/mol

mNaClO = 1.12 g

Preparing Solutions

Standard Solution is a solution for which the precise concentration is known. In the laboratory, standard solutions are often prepared in volumetric flasks. A volumetric flask is a pear shaped container with an elongated neck.  These are much more accurate than other flasks.

Solutions are prepared by first dissolving a known quantity of solute in about one quarter of the volume of the solvent, such as water (distilled).  The, additional solvent is carefully added until the bottom of the meniscus lines up with the calibration mark on the flask

Summary

The concentration of a solution is the quantity of dissolved solute per unit volume of solution.

Amount concentration is the amount (in moles) of solute dissolved per litre of solution.  The units of amount concentration are mol/L

Amount concentration is determined using the equation c = n / V.

“Amount concentration” is is the preferred IUPAC term for solution concentration (replacing molar concentration and molarity).

Samples taken from a stock solution are diluted to prepare solutions for use in the laboratory.

A solution of known concentration is called a standard solution.