A solution is prepared by dissolving 10.00g of a monobasic salt in a total volume of 1.00litres. Th pH of this solution was measured at 13.07 and a conductivity experiment determined that this salt was completely ionized in solution. What is the molecular weight of this monobasic salt?
Given:
Vtotal = 1.00 L
msalt = 10.00g
pHsol = 13.07
Know:
pH + pOH = 14.00
pOH = -log[ OH ^ - ]
C = n / V
n = m / M
Solve:
pH + pOH = 14.00
pOH = 14.00 - 13.07 <-- Since it is a monobasic salt
pOH = 0.93
pOH = -log[ OH ^ - ]
0.93 = -log[ OH ^ - ]
[ OH ^ - ] = 10^-(0.93)
[ OH ^ - ] = 0.117489 M = 0.117489 mol/L
C = n/v
n = (0.117489 mol/L)(1.00L)
n = 0.117489 mol
n = m/M
M = 10.00g / 0.117489 mol
M = 85.11380382 g/mol
M = 85.1 g/mol <-- 3 significant digits (check your givens, your volume should 1.00 L)
Therefor the molecular weight of this monbasic salt is 85.1 g/mol.