Question

A 4.36-g sample of an unknown alkali metal hydroxide is dissolved in 100.0 mL of water. An acid-base indicator is added and the resulting solution is titrated with 2.50 M HCl(aq)solution. The indicator changes color signaling that the equivalence point has been reached after 17.0 mL of the hydrochloric acid solution has been added.
A) what is the molar mass of the metal hydroxide?
B) what is the identity of the alkali metal cation: Li+, Na+, K+, Rb+, Cs+

Answer 1

Answer:

a. 102.6g/mol

b. Rb⁺

Explanation:

a. The unknown base, XOH, Where X is the alkali metal, reacts with HCl thus:

XOH + HCl → H₂O + XCl

Where 1 mole of the hydroxide reacts per mole of acid.

Moles of HCl spent to reach the equivalence point are:

17.0mL = 0.017L ₓ (2.50mol / L) = 0.0425moles of HCl

As 1 mole of HCl reacts with 1 mole of base, moles of base are 0.0425 moles.

Molar mass of a compound is the ratio between mass in grams and moles of substance, that is:

4.36g / 0.0425mol =

102.6g/mol

b) Molar mass of OH is 17g/mol, thus, the molar mass of the alkali metal is:

102.6g/mol - 17g/mol  = 85.6g/mol

The mass of Rb is 85.5g/mol, thus, the identity of the alkali metal is

Rb⁺