The mole concept and solutions
Francine Taylor-Campbell, Contributor

MAIN POINTS

• Concentration is expressed as the amount of solute in a given volume of solution.
• Concentration is expressed in units of moles of solute in 1dm³ of solution (mol/dm³).
• Mass concentration is the mass of the solute in 1dm3 of solution (g/dm³).
• A standard solution is a solution, the concentration of which, in mol/dm³ is known or can be calculated.

Question 1

A small piece of lithium of mass 0.35g is added to cold water. The resulting solution is titrated with 2.00 mol/dm³ hydrochloric acid.

(a) Write the equation between lithium and water and the lithium solution and hydrochloric acid.

(b) What volume of hydrochloric acid is needed to neutralise the lithium solution?

ANSWERS

(a) 2Li (s) + 2H₂O (l) == 2LiOH (aq) + H₂ (g)

Remember that the alkali metals dissolve in water to form a metal hydroxide and give off hydrogen.

LiOH (aq) + HCl (aq) == LiCl (aq) + H₂O (l)

(b) Molar mass of lithium is 7

# mol Li = 0.35/7 = 0.05 mol

Based on the equation Li reacts in a 1:1 ratio to form LiOH

Thus the # mol LiOH = 0.05 mol

LiOH and HCl also react in a 1:1 ratio

# mol of HCl is also 0.05 mol

Since 1000 cm³ contain 2.00 mol

X cm³ contain 0.05 mol thus X cm³ = (1000 x 0.05)/2 = 25 cm³

Francine Taylor-Campbell is an independent contributor.