The ionic compound sometimes called uranium yellow is used to produce colored glazes for ceramics. It is 7.252% sodium, 75.084% uranium, and 17.664% oxygen.
What is the empirical formula for this compound?
This compound has a molecular mass of 1268.06 g/mol, what is the molecular formula for this compound?
Take 100 g sample which gives you
7.252 g Na
75.084 g U
17.664 g O
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Convert to mols by mols = g/atomic mass.
mols Na = 7.252/23 = approx 0.315
mols U = 75.084/238.02 = approx 0.315
mols O = 17.664/16 = approx 1.104
Now convert these numbers into ratio to each element with the smallest number being 1.00. The easy way to do that is to divide the smallest number by itself and divide all of the other numbers by the same small number. That gives us
0.315/0.315 = 1.00 Na
0.315/0.315 = 1.00 U
1.10/0.315 = 3.50
USUALLY we round these to whole numbers; however, 0.5 is too much to round off BUT we see if we just multiplied all of the numbers by 2 we would get whole numbers so we have
2Na atoms
2U atoms
7 O atoms
and the empirical formula is Na2U2O7 for an empirical formula mass of
(2*23)+(2*238)+(7*16) = approx 634.
The molar mass is 1268; therefore, we have (empirical formula)*?number = 1268. Solve for the ? number and round it to a whole number. That's obviously 2 so the molecular formula is (Na2U2O7)2 or Na4U4O14.
To determine the empirical formula of a compound, you need to find the simplest ratio of atoms in the compound.
1. Start by assuming a 100g sample of the compound. This assumption allows you to work directly with percentages as grams.
2. Convert the percentages to grams by multiplying each percentage by 100g.
- Sodium: 7.252g
- Uranium: 75.084g
- Oxygen: 17.664g
3. Convert the grams of each element to moles by dividing each value by its respective atomic mass.
- Sodium: 7.252g / 22.99 g/mol = 0.315 moles
- Uranium: 75.084g / 238.0289 g/mol = 0.315 moles
- Oxygen: 17.664g / 16.00 g/mol = 1.104 moles
4. Find the smallest mole value (0.315) and divide all the moles by this value to obtain the simplest whole number ratio of atoms.
- Sodium: 0.315 moles / 0.315 moles = 1
- Uranium: 0.315 moles / 0.315 moles = 1
- Oxygen: 1.104 moles / 0.315 moles ≈ 3.50
Now, we have the empirical formula: NaUO₃.
To determine the molecular formula, we need to know the molecular mass of the empirical formula.
1. Add up the atomic masses of the empirical formula:
- Na: 22.99 g/mol
- U: 238.0289 g/mol
- O: 16.00 g/mol × 3 (since there are 3 oxygen atoms) = 48.00 g/mol
Total atomic mass of the empirical formula: 22.99 + 238.0289 + 48.00 = 309.0189 g/mol
2. Divide the molecular mass of the compound (1268.06 g/mol) by the empirical formula mass (309.0189 g/mol) to determine the molecular formula ratio.
- 1268.06 g/mol / 309.0189 g/mol ≈ 4.11
3. Multiply the subscripts in the empirical formula by the molecular formula ratio found in the previous step.
- Na: 1 × 4.11 = 4.11 ≈ 4
- U: 1 × 4.11 = 4.11 ≈ 4
- O: 3 × 4.11 = 12.33 ≈ 12
Therefore, the molecular formula for this compound is Na₄U₄O₁₂.