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 sol utio n 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 hydrochlor ic acid solution has been added. What is the molar mass of the metal hyd roxide?

Answer 1

We are given a sample of an unknown alkali metal hydroxide, and are being asked it's molar mass. If we were to consider this alkali metal hydroxide with HCl ( hydrochloric acid ), the following reaction would take place -

hydrochloric acid + sodium hydroxide → sodium chloride + water

Let me rewrite this reaction with respect to the " lettering " of the compounds,

HCl + MOH → MCl + H2O

M is replaced with the alkaline metal sodium in this case. From that, you can tell that this unknown alkali metal hydroxide is sodium hydroxide.

To solve for the number of moles of HCl, we can do the following -

n = C * V = 2.5 * 0.017 = 0.0425 moles,

n( MOH ) = n( HCl ) = 0.0425 moles

Using the formula n = m / M -

Molar Mass = m / n = 4.36 / 0.0425 ≈ 103 grams / mole

Solution = ( About ) 103 grams / moles of hydroxide