CHEM 24112 Lecture Notes - Lecture 24: Chemical Equation, Molar Mass, Empirical Formula

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29 Jun 2018

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CHEMISTRY 112 – DAYS 24 & 25 Chapter 8 – Sections 6-9 & Chapter 9 – Sections 1&2 3/12

& 3/14

MASS PERCENT

Mass percent of an element:

Mass percent for Carbon in C2H5OH?

EXAMPLE 1: Morphine, derived from opium plants, has the potential for use and abuse. Its formula is

C17H19NO3. What percent, by mass, is the carbon in this compound?

EXAMPLE 2: Consider separate 100.0 gram samples of each of the following: H2O, N2O, C3H6O2, CO2

Rank them from highest to lowest percent oxygen by mass.

EMPIRICAL FORMULAS

The empirical formula of a compound is the simplest whole number ratio of the atoms present in the

compound.

The empirical formula can be found from the percent composition of the compound.

Formulas

Empirical formula = CH -- Simplest whole-number ratio

Molecular formula = (empirical formula)n [n = integer]

Molecular formula = C6H6 = (CH)6

Actual formula of the compound

Steps for Determining the Empirical Formula of a Compound

A gaseous compound containing carbon and hydrogen was analyzed and found to consist of 83.65% carbon by

mass. Determine the empirical formula of the compound.

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Document Summary

Chemistry 112 days 24 & 25 chapter 8 sections 6-9 & chapter 9 sections 1&2 3/12. Example 1: morphine, derived from opium plants, has the potential for use and abuse. Example 2: consider separate 100. 0 gram samples of each of the following: h2o, n2o, c3h6o2, co2. Rank them from highest to lowest percent oxygen by mass. The empirical formula of a compound is the simplest whole number ratio of the atoms present in the compound. The empirical formula can be found from the percent composition of the compound. Empirical formula = ch -- simplest whole-number ratio. Steps for determining the empirical formula of a compound. A gaseous compound containing carbon and hydrogen was analyzed and found to consist of 83. 65% carbon by mass. Determine the empirical formula of the compound: obtain the mass of each element present (in grams). If all of the numbers so obtained are integers (whole numbers), these are the subscripts in the empirical formula.

Related Questions

Determine the empirical and molecular formulas of each of the following substances:

(a) Styrene, a compound substance used to make Styrofoam^® cups and insulation, contains 92.3% C and 7.7% H by mass and has a molar mass of 104 g/mol.

(b) Caffeine, a stimulant found in coffee, contains 49.5% C, 5.15% H, 28.9% N, and 16.5% O by mass and has a molar mass of 195 g/mol.

(c) Monosodium glutamate (MSG), a flavor enhancer in certain foods, contains 35.51% C, 4.77% H, 37.85% O, 8.29% N, and 13.60% Na, and has a molar mass of 169 g/mol.

Empirical formula of this compound is CH.

To determine the molecular formula, we need to find out how many times is molar mass higher than the mass of empirical formula:

Molecular formula is C₈H₈.

b) Empirical formula of caffeine:

Empirical formula of caffeine is C₄H₅N₂O. The sum of atomic masses from empirical formula is 97 g/mol.

Molecular formula of caffeine is 2 × empirical formula = C₈H₁₀N₄O₂

c) Empirical formula of MSG:

Empirical formula of MSG is C₅H₈NO₄Na. The sum of atomic masses of empirical formula is 169 g/mol, the same as molar mass. Empirical and molecular formula are identical.

cyanchimpanzee483

1) How many atoms are there in 51.3 g of barium chloride (BaCl2)?
2) How many moles of carbon monoxide (CO) are in 29.3 g of the compound?

3) What is the formula for copper(II) phosphate? Capitalization and punctuation count?

4) How many atoms of phosphorus are in 2.90 mol of copper(II) phosphate?

5) How many moles of N are in 0.191 g of N2O?

6) A sample of C3H8 has 7.68 Ã 1024 H atoms. How many carbon atoms does the sample contain? What is the total mass of the sample?

7) Calculate the percent composition by mass of each element in Al(OH)3. Use at least three significant figures.

% Al= ?, % O=? , % H =?

8) What is the empirical formula of a compound composed of 3.25% hydrogen (H), 19.36% carbon (C), and 77.39% oxygen (O) by mass?

HCO (Insert subscripts as needed.)

9) A compound with the empirical formula CH2 has a molar mass of 98 g/mol. What is the molecular formula for this compound?

Determine the empirical and molecular formulas of each of the following substances:
(a) Ibuprofen, a headache remedy, contains 75.69% C, 8.80% H, and 15.51% O by mass, and has a molar mass of 206 g/mol.
(b) Cadaverine, a foul-smelling substance produced by the action of bacteria on meat, contains 58.55% C, 13.81% H, and 27.40% N by mass; its molar mass is 102.2 g/mol.
(c) Epinephrine (adrenaline), a hormone secreted into the bloodstream in times of danger or stress, contains 59.0% C, 7.1% H, 26.2% O, and 7.7% N by mass; its MW is about 180 amu.

Empirial formula of ibuprofene is calculated by multiplying obtained ratio with 2, and the result is C₁₃H₁₈O₂. The sum of atomic mass of C₁₃H₁₈O₂ is 206 g/mol. Molecular formula is the same as empirical.

Empirical formula of cadaverine, C₅H₁₄N₂ is obtained by multiplying the moles of each atom with 2 in order to obtain the whole numbers. The weight of empirical formula is 102 g/mol, so it is at the same time the molecular formula.

c) 59.0% C, 7.1% H, 26.2% O, and 7.7% N

Empirical formula of adrenaline is C₉H₁₃NO₃, and its mass is 183 g/mol. Since MW of hormone is about 180, molecular formula is the same as empirical formula.

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CHEM 24112 Lecture Notes - Lecture 24: Chemical Equation, Molar Mass, Empirical Formula

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