A compound is analyzed and found to contain 68.54% carbon, 8.63% hydrogen, and 2

Question

A compound is analyzed and found to contain 68.54% carbon, 8.63% hydrogen, and 22.83% oxygen. The molecular weight of this compound is known to be approximately 140 g/mol. What is the empirical formula of the compound? What is the molecular formula of the compound? A graduated cylinder has a mass of 80 g when empty. When 20 mL of water is added. the graduated cylinder has a mass of 100 g. If a stone is added to the graduated cylinder, the water level rises to 45 mL d the total mass is now 0.156 kg. Calculate the density of the stone.

Explanation / Answer

(b)

(i) moles of C = 68.54/12 = 5.71 mol

moles of H = 8.63/1 = 8.63 mol

moles of O = 22.83/16 = 1.43 mol

divide by smallest factor

C = 5.71/1.43 = 4

H = 8.63/1.43 = 6

O = 1.43/1.33 = 1

Empirical formula = C4H6O

Emiprical formula weight = 12 x 4 + 6 x 1 + 16 = 70

(ii) formula weight ratio = 140/70 = 2

molecular formula = 2(C4H6O) = C8H12O2

(c) mass of stone = 156 - 80 = 76 g

volume of cullinder = 45 - 20 = 25 ml

density of stone = 76/25 = 3.04 g/ml

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