A sample of phosphorus of mass 3.654 g reacts with dichlorine to form 16.20 g of a molecular compound. What is the empirical formula of this compound?
P:
3.654 g P x (1 mol/30.97 g P) = .12 mol
Cl:
12.546 g Cl x (1 mol/35.45 g Cl) = .35 mol
Therefore, the empirical formula will be PCl3? Is this correct?
Compound of P and Cl = mass 16.20 g
Started with 3.654 g P = -3.654
Amount chlorine added = 16.20-3.654 = 12.546.
moles P = 3.546/molar mass P
moles Cl = 12.546/atomic mass Cl
Then find the molar ratios of the two elements with the smallest being 1.00. The easy way to do that is to divide the smallest number by itself thereby producing 1.00 for that element. Then divide the other number by the same small number. The numbers you get will be the x and y of PxCly.
Post your work if you get stuck.
I don't understand when you say divide by the smallest numbers. For instnace, am I supposed to divide 3.546 (P) by 3.546 again?
No. Did you convert g P to moles. Then convert g chlorine to moles. You want to find the molar ratios of THOSE two numbers (the moles).
Yes, but you rounded one of your numbers before you should have rounded.
3.546/30.97 = 0.1180 (don't round to 0.12--to two significant figures when you are allowed 4.
12.546/35.45 = 0.3539.
Your ratios are 0.12/0.12 = 1.00
0.35/0.12 = 2.92 so you rounded to P1Cl3 (PCl3) which is correct. Using my numbers, you would have obtained,
0.1180/0.1180 = 1.000
0.3539/0.1180 = 2.999
The final numbers are closer to whole numbers if you round correctly. In some cases it would make a difference in the formula obtained; in this case it did not.
To find the empirical formula of a compound, we need to determine the ratio of each element present in the compound. Here's how you can find the empirical formula using the given information:
1. Determine the number of moles of each element:
- Phosphorus: Divide the mass of phosphorus by its molar mass.
Molar mass of phosphorus (P) = 31.0 g/mol
Number of moles of phosphorus = mass of phosphorus / molar mass of phosphorus
- Chlorine: Divide the mass of chlorine by its molar mass.
Molar mass of chlorine (Cl) = 35.5 g/mol
Number of moles of chlorine = mass of chlorine / molar mass of chlorine
2. Find the ratio of the elements:
Divide the number of moles of each element by the smallest number of moles calculated in step 1. Round off the resulting values to the nearest whole number.
3. Write the empirical formula:
Use the ratios obtained in step 2 to write the empirical formula by providing the subscripts for each element.
Let's calculate it:
- Mass of phosphorus (P) = 3.654 g
- Number of moles of phosphorus = 3.654 g / 31.0 g/mol ≈ 0.118 mol
- Mass of chlorine (Cl) = 16.20 g - 3.654 g = 12.546 g
- Number of moles of chlorine = 12.546 g / 35.5 g/mol ≈ 0.353 mol
Now, let's determine the ratio:
- Ratio of phosphorus to chlorine ≈ 0.118 mol / 0.118 mol = 1
- Ratio of chlorine to phosphorus ≈ 0.353 mol / 0.118 mol ≈ 3
The empirical formula of the compound is PCl₃.