Gravimetric analysis does not always involve a precipitate. Consider the following problem. A mixture of calcium carbonate and calium oxide has a mass of 8.35 grams. When heated strongly, the CaCO3 in the sample decomposes into solid CaO and carbon dioxide gas. The calcium oxide in the original sample is not altered by heating. After heating, the mass of the sample is 6.37 grams. Using the information given, calculate the % by mass of calcium oxide in the original sample.
Write the equation for CaCO3 and balance it.
CaO in the sample is just an innocent bystander. Loss in weight on heating is due to CaCO3. How much did it lose? 8.35-6.37 = mass CO2.
Convert mass CO2 to mass CaCO3.
Then %CaCO3 = (mass CaCO3/mass sample)*100 = ??
%CaO = 100%-%CaCO3
To calculate the percentage by mass of calcium oxide in the original sample, we need to determine the mass of calcium oxide in the sample before heating and then divide it by the initial mass of the sample.
Here's how to solve the problem step by step:
1. Calculate the mass of the carbon dioxide gas that was evolved during the heating process:
- Mass of sample before heating: 8.35 grams
- Mass of sample after heating: 6.37 grams
- Mass of carbon dioxide evolved = Mass of sample before heating - Mass of sample after heating
= 8.35 g - 6.37 g
= 1.98 g
2. Determine the amount of calcium carbonate that decomposed and formed calcium oxide:
- The molar mass of CaCO3 = 40.08 g/mol + 12.01 g/mol + (3*16.00 g/mol) = 100.09 g/mol
- The molar mass of CO2 = 12.01 g/mol + (2*16.00 g/mol) = 44.01 g/mol
- The molar ratio of CO2 to CaCO3 is 1:1.
- Therefore, the amount of CaCO3 decomposed is equal to the amount of CO2 evolved.
- Amount of CaCO3 decomposed = Mass of CO2 evolved / Molar mass of CO2
= 1.98 g / 44.01 g/mol
3. Calculate the mass of calcium oxide formed:
- The molar mass of CaO = 40.08 g/mol + 16.00 g/mol = 56.08 g/mol
- The molar ratio of CaO to CaCO3 is 1:1.
- Therefore, the mass of CaO formed is equal to the mass of CaCO3 decomposed.
- Mass of CaO formed = Mass of CaCO3 decomposed
4. Determine the percentage by mass of calcium oxide in the original sample:
- Percentage by mass of CaO = (Mass of CaO formed / Mass of sample before heating) * 100
= (Mass of CaCO3 decomposed / Mass of sample before heating) * 100
Now you can substitute the values and calculate the final answer.