Question

6. Calculate the concentration of H3O + or HO- , as indicated, when each of the strong acids below is dissolved in the indicated total volume of water.

a) 1.5 moles of HBr is dissolved in 4.0 L of H2O. [H3O + ] = _____________ moles/L

b) 1.0 mole of Ba(OH)2 is dissolved in 10.0 L of H2O. [HO-] = ______________ moles/L

Answer 1

We need to calculate the Molarity of the solution. Remember that it is calculed by taking the moles of solute and dividing by the liters of solution.

a) 1.5 mol/4 L = 0.375 M = 0.375 mol/L

Stong acids (such as HBr) are strong electrolytes which are assumed to ionize completely in water.

The ionization of HBr is

HBr_(aq) + H₂O_(l) --> H₃O⁺_(aq) + Br-

The concentration of all the species before and after ionization can be represented as follows   

	HBr(aq)	-- >	H3O+(aq)	Br-
Initial (M)	0.375		0.0	0.0
Chage (M)	-0.375		+0.375	+0.375
Equilibrium (M)	0		0.375	+0.375

Thus [H3O + ] = 0.375 mol/L

b) 1 mol/10 L = 0.1 M

Stong bases are strong electrolytes that ionize completely in water.

Ba(OH)₂ is a strong base; each Ba(OH)₂ unit produces two OH^- ions:

Ba(OH)_2(aq) --> Ba²⁺_(aq) + 2 OH^-_(aq)

The changes in concentration of all species can be represented as follows:

	Ba(OH)2(aq)	-- >	Ba2+(aq)	2 OH-(aq)
Initial (M)	0.1		0.0	0.0
Chage (M)	-0.1		+0.1	+2 (0.1)
Equilibrium (M)	0		0.1	0.2

Thus

[HO-] = 0.2 M = 0.2 mol/L