Paper 3B problems

# Percentage Purity

I have a sample of sodium chloride which has some limestone powder in it. How would I find out the % of the impurity?HINT:solubility
The sodium chloride is soluble in water but limestone is insoluble so this problem is a "salt" and sand excercise.

• weigh sample
• add water to dissolve the sodium chloride
• filter
• dry the residue to constant weight

*mass impurity/total mass x 100=%impurity

How could I determine the % of sodium carbonateimpurity in a sample of Sodium chloride?HINT:all sodium salts are soluble

• weigh sample
• place in a conical flask connected to a gas syringe

OR measure mass loss on a balance

• collect gas given off
• moles carbon dioxide=volume/24000

OR mass carbon dioxide given off/44

• moles carbon dioxide = moles sodium carbonate
• RMM sodium carbonate = 106 so mass sodium carbonate = 106 x moles sodium carbonate
• % impurity= mass sodium caronate/original mass x100

How could I find the % sodium nitrate impurity in a sample of sodium chloride?
HINT:neither salt reacts with acids

• weigh sample
• add excess aqueous silver nitrate solution and nitric acid
• filter off the Silver Chloride ppt, dry to constant weight
• RMM AgCl = 108 +35.5 = 143.5 so
• moles AgCl = mass ppt/143.5
• moles AgCl = moles NaCl so
• Mass NaCl=moles x 58.5
• total mass - mass NaCl = mass sodium nitrate
• mass sodium nitrate/total mass x 100 = %impurity
page revision: 3, last edited: 07 May 2008 12:52