1a. The empirical formula of the compound is Fe₂O₃
1b. The molecular formula of the compound is Fe₂O₃
2a. The molecular formula of the anhydrous salt is MgSO₄
2b. The formula of the crystalline salt is MgSO₄.7H₂O
3i. The number of mole of carbon atoms in the compound is 6 moles
3ii. The number of mole of hydrogen atoms in the compound is 18 moles
3iii. The number of molecules in 3 moles of ethane is 1.806×10²⁴ molecules
1a. How to determine the empirical formula
- Fe = 69.9%
- O = 30%
- Empirical formula =?
Divide by their molar mass
Fe = 69.9 / 56 = 1.248
O = 30 / 16 = 1.875
Divide by the smallest
Fe = 1.248 / 1.248 = 1
O = 1.875 / 1.248 = 3/2
Multiply by 2 to express in whole number
Fe = 1 × 2 = 2
O = 3/2 × 2 = 3
Thus, the empirical formula of the compound is Fe₂O₃
1b. How to determine the molecular formula
- Empirical formula = Fe₂O₃
- Molar mass of compound = 159.89 g/mol
- Molecular formula = ?
Molecular formula = empirical × n = molar mass
[Fe₂O₃]n = 159.89
[(56×2) + (16×3)]n = 159.89
160n = 159.89
n = 159.89 / 160
n = 1
Molecular formula = [Fe₂O₃]n
Molecular formula = [Fe₂O₃] × 1
Molecular formula = Fe₂O₃
2a. How to determine the molecual formula of the anhydrous salt
We'll begin by calculating the empirical formula
- Mg = 20.0%
- S = 26.66%
- O = 53.33%
- Empirical formula =?
Divide by their molar mass
Mg = 20.0 / 24 = 0.83
S = 26.66 / 32 = 0.83
O = 53.33 / 16 = 3.33
Divide by the smallest
Mg = 0.83 / 0.83 = 1
S = 0.83 / 0.83 = 1
O = 3.33 / 0.83 = 4
Thus, the empirical formula of the anhydrous salt is MgSO₄
The molecular formula of the anhydrous salt can be obtained as follow:
- Empirical formula = MgSO₄
- Molar mass of compound = 120 g/mol
- Molecular formula = ?
Molecular formula = empirical × n = molar mass
[MgSO₄]n = 120
[24 + 32 + (16×4)]n = 159.89
120n = 120
n = 120 / 120
n = 1
Molecular formula = [MgSO₄]n
Molecular formula = [MgSO₄] × 1
Molecular formula = MgSO₄
2b. How to determine the formula of the crystalline salt
- Water (H₂O) = 51.2%
- Anhydrous salt (MgSO₄) = 100 - 51.2 = 48.8%
- Formula of crystalline salt =?
Divide by their molar mass
MgSO₄ = 48.8 / 120 = 0.4
H₂O = 51.2 / 18 = 2.8
Divide by the smallest
MgSO₄ = 0.4 / 0.4 = 1
H₂O = 2.8 / 0.4 = 7
Thus, the formula of the crystalline salt is MgSO₄.7H₂O
3i. How to determine the mole of carbon atoms in 3 moles of C₂H₆
1 mole of C₂H₆ contains 2 moles of carbon atoms.
Therefore,
3 moles of C₂H₆ will contain = 3 × 2 = 6 moles of carbon atoms
3ii. How to determine the mole of hydrogen atoms in 3 moles of C₂H₆
1 mole of C₂H₆ contains 6 moles of hydrogen atoms.
Therefore,
3 moles of C₂H₆ will contain = 3 × 6 = 18 moles of hydrogen atoms
3iii. How to determine the number of molecules
From Avogadro's hypothesis,
1 mole of ethane = 6.02×10²³ molecules
Therefore,
3 moles of ethane = 3 × 6.02×10²³ molecules
3 moles of ethane = 1.806×10²⁴ molecules
Learn more about empirical formula:
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Learn more about Avogadro's number:
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Complete question
2. A crystalline salt when heated becomes anhydrous and loses 51.2% of its weight the anhydrous salt analysis gave the percent composition as magnesium is equal to 20.0% and sulphur is equal to 26.66% and oxygen is equal to 53.33%. Ccalculate the molecular formula of the anhydrous and the crystalline salt. The molecular weight of the anhydrous salt is 120
