Elemental Analysis
Analyzing the the composition
- Elemental analysis is a technique that can be used to determine the relative composition of elements in a substance and evaluate its purity
Worked example
A 4.1 g sample of a mixture of calcium chloride (CaCl2) and magnesium fluoride (MgF2) are found to contain 0.30 g of Ca. What is the percent of CaCl2 in the sample?
Answer
- Step 1: Determine the moles of Ca in the sample
- n = m / M
- n = 0.30 g / 40.08 g mol-1
- n = 0.00749 mol of Ca
- Step 2: Determine the moles of CaCl2 using the moles of Ca
- The subscripts show that there is one atom of Ca in one formula unit of CaCl2. Therefore,
- The subscripts show that there is one atom of Ca in one formula unit of CaCl2. Therefore,
- Step 3: Determine the mass of CaCl2 in the sample by rearranging
- n = m / M
- m = n x M
- m = 0.749 mol x 110.98 g mol-1
- m = 0.83 g of CaCl2
- Step 4: Calculate the percent of CaCl2 in the sample
- percent of CaCl2 = (mass of CaCl2 / total mass of sample) × 100
- percent of CaCl2 = (0.83 g of CaCl2 / 4.1 g of sample) × 100
- percent of CaCl2 = 20%
Analyzing the purity
- Purity is the extent to which a substance contains only a desired chemical compound without any foreign substances
- Purity is often shown as a percentage
- It can be calculated either using the mass (m) of the desired compound or its % composition
Worked example
5.00 g of a sodium chloride sample were analyzed and it was found that it was contaminated with sulfur. The sample is determined to contain 1.35 g of sodium, 2.00 chlorine, and the rest is sulfur. What is the purity of the sample?
Answer
- Step 1: Identify the desired compound
- The sample was intended to be sodium chloride so, NaCl is our desired compound
- Step 2: Calculate the mass of the desired compound
- The statement lists the masses of sodium and chlorine. Since these are the atoms that are part of NaCl, we add both masses
- m of desired compound = m of Na + m of Cl
- m of desired compound = 1.35 g + 2.00 g
- m of desired compound = 3.35 g
- Step 3: Determine the purity using formula that depends on m of desired composition and the total mass of sample