Since soap and detergent action is hindered by hard water, laundry formulations
ID: 504575 • Letter: S
Question
Since soap and detergent action is hindered by hard water, laundry formulations usually include water softeners-called builders-designed to remove hard water ions (especially Ca2+ and Mg2+) from the water. A common builder used in North America is sodium carbonate. Suppose that the hard water used to do laundry contains 75 ppm CaCO3 and 55 ppm MgCO3 (by mass).
What mass of Na2CO3 is required to remove 90.0% of these ions from 10.0 L of laundry water?
Answers on other posts that have been incorrect: 1.3g, 1.4g, .668g
Explanation / Answer
1ppm = 1mg / Kg
The concentration of CaCO3 = 75ppm = 75mg / 10^3 grams of water
and MgCO3 = 55ppm = 55mg / 10^3 grams of water
so in each 10,000 grams of water
Mass of CaCO3 = 750mg
Mass of MgCO3 =550mg
we have to remove 90% of these ions then,
the mass to be removed = 0.90 X 750 mg = 675 mg CaCO3
Moles to be removed = Mass / Molecular weight = 0.675 / 100 = 0.00675 moles
The mass to be removed =0.90 X 550 mg = 495mg of MgCO3
Moles to be removed = 0.495 / 84.3 = 0.00587 moles
Now Na2CO3 is used to remove hardness to non carbonate ions of calcium and magnesium
so it will not be able to remove the CaCO3 and MgCO3 hardness.
We need lime to remove the given hardness in water.