General orrganic and biological chemistry structures off liffe 5th by karen timberlake07 worked examples

Sample Problem 7.1 Number of Atoms in Balanced Chemical Equations
Indicate the number of each type of atom in the following balanced chemical equation:

Fe2S3(s) + 6HCl(aq) → 2FeCl3(aq) + 3H2S(g)

Solution
The total number of atoms in each formula is obtained by multiplying the coefficient by each subscript in a chemical formula.

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Sample Problem 7.1 Number of Atoms in Balanced Chemical Equations
Continued

Study Check 7.1
When ethane, C2H6, burns in oxygen, the products are carbon dioxide and water. The balanced chemical equation is written as

Δ
2C2H6(g) + 7O 2(g) → 4CO2(g) + 6H2O(g)

Calculate the number of each type of atom in the reactants and in the products.

Answer
In both the reactants and products, there are 4 C atoms, 12 H atoms, and 14 O atoms.

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Sample Problem 7.2 Balancing a Chemical Equation
The chemical reaction of methane, CH 4, and oxygen gas, O 2, produces carbon dioxide (CO2) and water (H2O). Write a balanced chemical equation for this reaction.

Solution

Step 1

Write an equation using the correct formulas for the reactants and products.

Δ
CH4(g) + O2(g) → CO2(g) + H2O(g)

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Sample Problem 7.2 Balancing a Chemical Equation
Continued
Step 2

Count the atoms of each element in the reactants and products. When we compare the atoms on the reactant side and the atoms on the product side, we see that there are more H
atoms in the reactants and more O atoms in the products.

Step 3

Use coefficients to balance each element. We will start by balancing the H atoms in CH 4 because it has the most atoms. By placing a coefficient of 2 in front of the formula for
H2O, a total of 4 H atoms in the products is obtained. Only use coefficients to balance an equation. Do not change any of the subscripts: This would alter the chemical formula of
a reactant or product.

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Sample Problem 7.2 Balancing a Chemical Equation
Continued
We can balance the O atoms on the reactant side by placing a coefficient of 2 in front of the formula

Step 4

O 2. There are now 4 O atoms in both the reactants and products.

Check the final equation to confirm it is balanced. In the final equation, the numbers of atoms of C, H, and O are the same in both the reactants and the products. The equation is
balanced.

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Sample Problem 7.2 Balancing a Chemical Equation
Continued
In a balanced chemical equation, the coefficients must be the lowest possible whole numbers. Suppose you had obtained the following for the balanced equation:

Although there are equal numbers of atoms on both sides of the equation, this is not written correctly. To obtain coefficients that are the lowest whole numbers, we divide all the
coefficients by 2.

Study Check 7.2
Balance the following chemical equation:

Al(s) + Cl2(g) → AlCl3(s)

Answer

2Al(s) + 3Cl2(g) → 2AlCl3(s)

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Sample Problem 7.3 Balancing Chemical Equations with Polyatomic Ions
Balance the following chemical equation:

Na3PO4(aq) + MgCl2(aq)2 → Mg3(PO4)2(s) + NaCl(aq)

Solution

Step 1

Write an equation using the correct formulas for the reactants and products.

Na3PO4(aq) + MgCl2(aq) → Mg3(PO4)2(s) + NaCl(aq)

Step 2

Unbalanced

Count the atoms of each element in the
reactants and products. When we compare
the number of ions in the reactants and products,
we find that the equation is not balanced. In this
equation, we can balance the phosphate ion as a
group of atoms because it appears on both sides
of the equation.

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Sample Problem 7.3 Balancing Chemical Equations with Polyatomic Ions
Continued
Na3PO4(aq) + MgCl2(aq) → Mg3(PO4)2(s) + NaCl(aq)

Step 3

Use coefficients to balance each element. We begin with the formula that has the highest subscript values, which in this equation is Mg 3(PO4)2. The subscript 3 in Mg3(PO4)2 is
used as a coefficient for MgCl2 to balance magnesium. The subscript 2 in Mg 3(PO4)2 is used as a coefficient for Na3PO4 to balance the phosphate ion.

2Na3PO4(aq) + 3MgCl2(aq) → Mg3(PO4)2(s) + NaCl(aq)

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Sample Problem 7.3 Balancing Chemical Equations with Polyatomic Ions
Continued
In the reactants and products, we see that the sodium and chloride ions are not yet balanced. A coefficient of 6 is placed in front of the NaCl to balance the equation.

2Na3PO4(aq) + 3MgCl2(aq) → Mg3(PO4)2(s) + 6NaCl(aq)

Step 4

Check the final equation to confirm it is balanced. A check of the total number of ions confirms the equation is balanced. A coefficient of 1 is understood and not usually written.

2Na3PO4(aq) + 3MgCl2(aq) → Mg3(PO4)2(s) + 6NaCl(aq)

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

Balanced

© 2016 Pearson Education, Inc.


Sample Problem 7.3 Balancing Chemical Equations with Polyatomic Ions
Continued

Study Check 7.3
Balance the following chemical equation:

Pb(NO3)2(aq) + AlBr3(aq) → PbBr2(s) + Al(NO3)3(aq)

Answer
3Pb(NO3)2(aq) + 2AlBr3(aq) → 3PbBr2(s) + 2Al(NO3)3(aq)

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Sample Problem 7.4 Identifying Reactions and Predicting Products
Classify each of the following as a combination, decomposition, single replacement,
double replacement, or combustion reaction:
a.

2Fe2O3(s)+ 3C(s) → 3CO2(g) + 4Fe(s)
Δ

b.

2KClO3(s) → 2KCl(s) + 3O2(g)
Δ

c.

C H (g) + 3O 2(g) → 2CO2(g) + 2H2O(g) + energy

a.

In this single replacement reaction, a C atom replaces Fe in Fe 2O3 to form the compound CO 2 and Fe atoms.

b.

When one reactant breaks down to produce two products, the reaction is decomposition.

c.

The reaction of a carbon compound with oxygen to produce carbon dioxide, water, and energy makes this a combustion reaction.

2 4
Solution

Study Check 7.4
Nitrogen gas (N2) and oxygen gas (O 2) react to form nitrogen dioxide gas. Write the balanced chemical equation using the correct chemical formulas of the reactants and product, and identify the
reaction type.

Answer
N2(g) + 2O2(g) → 2NO2(g)

combination

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.

Sample Problem 7.5 Calculating the Number of Molecules
How many molecules are present in 1.75 moles of carbon dioxide, CO 2?

The solid form of carbon dioxide is known as “dry ice.”

Solution

Step 1

State the given and needed quantities.

Step 2

Write a plan to convert moles to atoms or molecules.

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Sample Problem 7.5 Calculating the Number of Molecules
Continued
Step 3

Use Avogadro’s number to write conversion factors.

Step 4

Set up the problem to calculate the number of particles.

Study Check 7.5
How many moles of water, H2O, contain 2.60 × 10 23 molecules of water?

Answer
0.432 mole of H2O

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Sample Problem 7.6 Calculating the Moles of an Element in a Compound
Propyl acetate, C5H10O2, gives the odor and taste to pears. How many moles of C are present in 1.50 moles of propyl acetate?

Solution

Step 1

State the given and needed quantities.

Step 2

Write a plan to convert moles of compound to moles of an element.

Step 3

Write equalities and conversion factors using subscripts.

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Sample Problem 7.6 Calculating the Moles of an Element in a Compound
Continued
Step 4

Set up the problem to calculate the moles of an element.

Study Check 7.6
How many moles of propyl acetate, C5H10O 2, contain 0.480 mole of O?

Answer
0.240 mole of propyl acetate

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Sample Problem 7.7 Calculating the Molar Mass for a Compound
Calculate the molar mass for lithium carbonate, Li 2CO3, used to treat bipolar disorder.

Solution

Step 1

Obtain the molar mass of each element.

Step 2

Multiply each molar mass by the number of moles (subscript) in the formula.
Grams from 2 moles of Li:

Grams from 1 mole of C:

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Sample Problem 7.7 Calculating the Molar Mass for a Compound
Continued
Grams from 3 moles of O:

Step 3

Calculate the molar mass by adding the masses of the elements.
2 moles of Li

= 13.88 g of Li

1 mole of C

= 12.01 g of C

3 moles of O

= +48.00 g of O

Molar mass of Li2CO3

= 73.89 g

Study Check 7.7
Calculate the molar mass for salicylic acid, C7H6O3, used to treat skin conditions such as acne, psoriasis, and dandruff.

Answer
138.12 g

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Sample Problem 7.8 Converting Moles of an Element to Grams
Silver metal is used in the manufacture of tableware, mirrors, jewelry, and dental alloys. If the design for a piece of jewelry requires 0.750 mole of silver, how many grams of silver are needed?

Solution

Step 1

State the given and needed quantities.

Step 2

Write a plan to convert moles to grams.

Step 3

Determine the molar mass and write conversion factors.

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Sample Problem 7.8 Converting Moles of an Element to Grams
Continued
Step 4

Set up the problem to convert moles to grams. Calculate the grams of silver using the molar mass factor that cancels mole Ag.

Study Check 7.8
A dentist orders gold to prepare dental crowns and fillings. Calculate the number of grams of gold (Au) present in 0.124 mole of gold.

Answer
24.4 g of Au

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Sample Problem 7.9 Converting the Mass of a Compound to Moles
A box of salt contains 737 g of NaCl. How many moles of NaCl are present?

Solution

Step 1

State the given and needed quantities.

Step 2

Write a plan to convert grams to moles.

Step 3

Determine the molar mass and write conversion factors.
(1 × 22.99) + (1 × 35.45) = 58.44 g/mole

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Sample Problem 7.9 Converting the Mass of a Compound to Moles
Continued

Step 4

Set up the problem to convert grams to moles.

Study Check 7.9
One tablet of an antacid contains 680. mg of CaCO 3. How many moles of CaCO 3 are present?

Answer
0.00679 mole of CaCO3

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Sample Problem 7.10 Calculating Moles of a Reactant
In the chemical reaction of iron and sulfur, how many moles of sulfur are needed to react with 1.42 moles of iron?

2Fe(s) + 3S(s) → Fe2S3(s)

Solution

Step 1

State the given and needed quantities (moles).

Step 2

Write a plan to convert the given to the needed quantity (moles).

Step 3

Use coefficients to write mole–mole factors.

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Sample Problem 7.10 Calculating Moles of a Reactant
Continued
Step 4

Set up the problem to give the needed quantity (moles).

Study Check 7.10
Using the equation in Sample Problem 7.10, calculate the number of moles of iron needed to react with 2.75 moles of sulfur.

Answer
1.83 moles of iron

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Sample Problem 7.11 Mass of Product

When acetylene, C2H2, burns in oxygen, high temperatures are produced that are used for welding metals.

Δ
2C2H2(g) + 5O2(g) → 4CO2(g) + 2H2O(g)

How many grams of CO 2 are produced when 54.6 g of C2H2 is burned?

Solution

Step 1

State the given and needed quantities (grams).

Step 2

Write a plan to convert the given to the needed quantity (grams).

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Sample Problem 7.11 Mass of Product
Continued
Step 3

Use coefficients to write mole–mole factors; write molar mass factors if needed.

Step 4

Set up the problem to give the needed quantity (grams).

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.