username1673629
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#1
Report Thread starter 6 years ago
#1
Hi, I've been given a question on formulas as a homework task and I think I have the answer but we haven't covered this type of question yet in class so I could do with someone checking it for me please.

The question is:
A substance contains carbon, hydrogen, and oxygen. Its relative molecular mass is 85. When burnt, 0.43 grams of the substance produces 1.10 grams of carbon dioxide and 0.45 grams of water. (a) Find the empirical formula and (b) the molecular formula.

My answer for both is C₅H₁₀O

Here's my working:
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a. Calculate the empirical formula.
Step 1 - Masses of substances involved
CO₂ - 1.10 grams, H₂O - 0.45 grams, CxHxOx - 0.43

Step 2 - Molecular masses of substances involved
CO₂ – 44, H₂O – 18, CxHxOx – 85

Step 3 – Amount of Carbon, Hydrogen, and Oxygen
1.1 x (12 ÷ 44) = 0.3 grams of Carbon in CO₂
0.45 x (2 ÷ 18) = 0.05 grams of Hydrogen in H₂O
0.43 – (0.3 + 0.05) = 0.08 grams of O in CxHxOx

Step 4 – Moles
0.3 ÷ 12 = 0.025 moles of carbon
0.05 ÷ 1 = 0.05 mole of hydrogen
0.08 ÷ 16 = 0.005 moles of oxygen

Step 5 – Ratio
5:10:1

C₅H₁₀O

b. Calculate the molecular formula
Step 1 – Calculate mass of empirical formula
(5 x 12) + (10 x 1) + (1 x 16) = 86 grams

Step 2 – Calculate how many empirical units are in the molecular formula
86 / 85 = 1.01 ≈ 1

C₅H₁₀O


Thanks for any input
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