Question

Can someone explain the logic behind this question?

The addition of 1 g of which of the following compounds to distilled water would cause the least change in pH?

a) Cs(OH)

b)HCl

c) LiH

d) LiOH

I know that HCL is a strong acid which elimates that since it will affect PH. In class we had to memorize that group I metals with OH are strong bases making which LiOH and CsOH follow. The answer is A. The solution to my homework said it has to do with radius can someone explain this to me? I would have picked LiH since it is not classified as a strong base from what we had to memorize even though there are many reations in which Hydride ions act as a base.

Thank You

Answer 1

Because all compounds are strong electolytes (complete dissociation), the compound with the highest molecular weight will have the lowest molar concentration of H+ or OH- and thus cause the least change in pH.

Suppose that assume we have 1 L of distilled water now calculate molarity of each of above 1g compound

1. CsOH molar mass = 149.912 g/mol

Molarity = (mass of substance/molar mass of substance) x 1/ volume in liters

            = (1g/149.912 g/mol)x 1/1 =                                                                             0.00667 M

2. HCl molarity = 1g/36.46 g/mol =                                                                                0.02743 M

3. Lithium hydride reacts with water vigorously as LiH + H2O --> LiOH + H2

1mol LiH gives 1mol LiOH, hence mass of LiOH formed is = 0.1258 mol/liter =                   0.1258 M

4. 1g LiOH molarity =                                                                                                   0.04175 M

out of above four only CsOH molar concentration is very very less, hence it would cause the least change in pH.

Answer: A) CsOH

Thank You!

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