GRAVIMETRIC ANALYSIS OF

WATER HARDNESS

Have you ever noticed this?

“Hard” water has ions that are dissolved in it, but can eventually combine with other ions to form a solid, and precipitate out of the water (and on to your faucets).

Why is our water“hard”?

Where does our water come from?

Where does our water come from? lakes

Lakes are surrounded by rock, so calcium chloride and other minerals dissolve into the water.

Where does our water come from? lakes

Lakes are surrounded by rock, so calcium chloride and other minerals dissolve into the water.

Isn’t our water treated before drinking?

Where does our water come from? lakes

Lakes are surrounded by rock, so calcium chloride and other minerals dissolve into the water.

Isn’t our water treated before drinking?It is treated for bacteria and other things that can make us sick (mainly with chlorine), but it is not purified.

So, how hard is our water?

So, how hard is our water? Let’s find out.

In this lab we will measure the amount of calcium in the water and compare that to a “water hardness” scale.

But first, we’ll find the calcium in a standard solution to practice our techniques and determine how well they work.

GRAVIMETRIC ANALYSIS OF A

STANDARD SOLUTION

We will be starting with 20 mL of a 1.0 molar solution of calcium chloride and mixing it with 20 mL of a 1.0 molar solution of sodium carbonate.

When we mix 20 mL of a clear solution of 1.0 molar calcium chloride and 20 mL of a clear solution of 1.0 molar sodium carbonate, then we should get a solution containing a white, solid precipitate of calcium carbonate.

And then real life gets in the way of chemistry…

We rarely get the full amount of products. To know how well our reaction actually worked, we calculate a percent yield.

% yield =Actual (measured) Amount

X 100Theoretical (calculated) Amount

Our desired product is a white solid floating in our clear solution. How can we easily separate a solid from a liquid?

Our desired product is a white solid floating in our clear solution. How can we easily separate a solid from a liquid?

Filtration!

But we want to be able to measure the mass of our solid (precipitate) that gets caught on the filter, so we have to do some preparation.

What should be done to the filter before we use it?

But we want to be able to measure the mass of our solid (precipitate) that gets caught on the filter, so we have to do some preparation.

What should be done to the filter before we use it?

Write your name on itFind the mass of it

So we have the mass of the filter alone.

Then we filter.

Then we have our precipitate caught on the filter, but there is a problem.

So we have the mass of the filter alone.

Then we filter.

Then we have our precipitate caught on the filter, but there is a problem.

The filter will be wet. So it will have to dry before we measure the mass.

GRAVIMETRIC ANALYSIS OF A

UNKNOWN SOLUTION

Water of unknown calcium chloride content = tap water.

Water of unknown calcium chloride content = tap water.

We will repeat the steps as with the standard solutions, but will need some additional calculations:

Water of unknown calcium chloride content = tap water.

We will repeat the steps as with the standard solutions, but will need some additional calculations:•Calculate theoretical amount, in molarity

Water of unknown calcium chloride content = tap water.

We will repeat the steps as with the standard solutions, but will need some additional calculations:•Calculate theoretical amount, in molarity•Convert molarity to Parts Per Million (ppm)

Water of unknown calcium chloride content = tap water.

We will repeat the steps as with the standard solutions, but will need some additional calculations:•Calculate theoretical amount, in molarity•Convert molarity to Parts Per Million (ppm)

Then we can compare to a chart and see how hard our water is.