H2 Chemistry 6 - Mole Concept and Stoichiometry: Back Titration

Step 1 - State the problem

A sample of impure calcium carbonate of mass two point five zero grams is dissolved in fifty point zero cubic centimetres of one point zero zero molar hydrochloric acid.

Step 1 - State the problem

The excess acid is then titrated with zero point five zero zero molar sodium hydroxide, requiring twenty point zero cubic centimetres to reach the endpoint.

Step 1 - State the problem

Calculate the percentage purity of the calcium carbonate sample.

Step 2 - Calculate the initial moles of HCl

First, calculate the moles of hydrochloric acid added initially.

Step 2 - Calculate the initial moles of HCl

Moles equals concentration times volume in cubic decimetres.

Step 2 - Calculate the initial moles of HCl

Fifty cubic centimetres is zero point zero five zero zero cubic decimetres.

Step 2 - Calculate the initial moles of HCl

So moles of HCl equals one point zero zero times zero point zero five zero zero, which is zero point zero five zero zero moles.

Step 3 - Calculate the moles of excess HCl from the back titration

The excess HCl reacts with sodium hydroxide in a one-to-one ratio.

Step 3 - Calculate the moles of excess HCl from the back titration

Moles of NaOH equals zero point five zero zero times zero point zero two zero zero, which is zero point zero one zero zero moles.

Step 3 - Calculate the moles of excess HCl from the back titration

Therefore, moles of excess HCl is also zero point zero one zero zero moles.

Step 4 - Find moles of HCl that reacted with calcium carbonate

The moles of HCl that reacted with calcium carbonate equals the initial moles minus the excess.

Step 4 - Find moles of HCl that reacted with calcium carbonate

That is zero point zero five zero zero minus zero point zero one zero zero, which equals zero point zero four zero zero moles.

Step 4 - Find moles of HCl that reacted with calcium carbonate

From the equation, calcium carbonate reacts with HCl in a one-to-two ratio.

Step 4 - Find moles of HCl that reacted with calcium carbonate

So moles of calcium carbonate equals zero point zero four zero zero divided by two, which is zero point zero two zero zero moles.

Step 5 - Calculate percentage purity

The mass of pure calcium carbonate equals moles times molar mass.

Step 5 - Calculate percentage purity

The molar mass of CaCO three is forty point one plus twelve point zero plus three times sixteen point zero, which equals one hundred point one grams per mole.

Step 5 - Calculate percentage purity

Mass equals zero point zero two zero zero times one hundred point one, which is two point zero zero grams.

Step 5 - Calculate percentage purity

Percentage purity equals two point zero zero divided by two point five zero, times one hundred percent, which gives eighty point zero percent.