Calculate the molecular formula for caffeine, a compound with a molar mass of 19

Question

Calculate the molecular formula for caffeine, a compound with a molar mass of 195.0 g and the following percent composition: 49.5% C, 5.15% H, 28.9% N and 16.5% o. A compound used as a dry-cleaning fluid was analyzed and found to contain 18.0% C, 2.27% H and 79.73% Cl. Determine its empirical formula. If the molar mass of this compound was found to be 266.8 g/mol, what would be the molecular formula of this compound? A 2.554 g sample of a certain hydrocarbon is burned in excess oxygen, producing 8.635 g of gaseous CO2 and 1.768 g of liquid H,O. If the molecular weight of the hydrocarbon is found to be 78.11 amu, what is its molecular formula? 7) 8) 9) 10) A 0.77 mg sample of nitrogen reacts with chlorine to form 6.61 mg of the chloride. Determine the empirical formula of nitrogen chloride. 11) A 45.2 mg sample of phosphorous reacts with selenium to form 131.6 mg of the selenide. Determine the empirical formula of phosphorous selenide

Explanation / Answer

Answer:

Start with the number of grams of each element given in the problem.

As we know in question percentage are given and assume that the total mass is 100 grams so that the mass of each element = the percent given.

Given:

Carbon = 49.5 %

Hydrogen = 5.15 %

Nitrogen = 28.9 %

Oxygen = 16.5 %

Convert the mass of each element to moles using the molar mass from the help of periodic table. (Slash is used for upon)

49.5 g C × 1 mol C/12 g C = 4.125 mol C

5.15 g H × 1 mol H /1 g H = 5.15 mol H

28.9 g N × 1 mol N/14 g N = 2.064 mol N

16.5 g O × 1 mol O/16 g O = 1.031 mol O

Divide each mol value by the smallest number of moles, and calculated round to the nearest whole number.

Carbon: 4.125/1.031 = 4

Hydrogen: 5.15/1.031 = 5

Nitrogen: 2.064/1.031 = 2

Oxygen: 1.031/1.031 = 1

This is the mole ratio of the elements and represented by the subscript in the empirical formula.

Empirical Formula: C4H5N2 O

Molar mass given = 195.0

Now, we can find the molecular formula by finding the mass of the empirical formula and setting up a ratio:

Molar mass /Empirical mass = 195g per mol/97 g per mol = 2.010

So the molecular formula is the double in the ratio of empirical formula

Molecular Formula of Caffeine: C8H10N4O2

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