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Coyle/Tang-Johnson Chemistry Study Guide

Chapter 10 Study Guide- Chemical QuantitiesTEST: Thursday 1/27/11

10.1 The Mole: Measurement of Matter

Define the following terms:

- mole

- Avogadro’s number

- representative particle

- molar mass

1. What kind of representative particles can one calculate using Avogadro’s number?

Converting Moles to # of Particles Converting # of Particles to Moles

1 Mole = 6.02 x 1023 particles

therefore

1 mol or 6.02 x 10 23 particles 6.02 x 1023 particles 1 mol

(moles particles)

If you are given moles and you want to find particles then you multiply by 6.02 x 1023

particles.

(particles moles)

If you are given particles and want to find the moles you divide by 6.02 x 1023

particles.

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If you want to find the # of atoms you first find the # of particles or molecules and then multiply by the # of atoms per molecule. (mol particles atoms)

Example: How many oxygen atoms are there in 5 moles of NO3?

1) Because NO3 is a molecule, first find the number of particles of NO3 in 5 moles.

5 mol NO3 x 6.02 x 10 23 molecules NO 3 = 3.01 x 1024 molecules NO3 1 mol NO3

2) Next, figure out how many oxygen atoms there are in NO3. There are 3

oxygen atoms in every molecule of NO3

3) Now set up your equation:

3.01 x 1024 molecules NO3 x 3 oxygen atoms = 9.03 x 1024 oxygen atoms 1 molecule NO3

If you want, you can set it up as one big conversion rather than in 2 steps:

5 mol NO3 x 6.02 x 10 23 molecules NO 3 x 3 atoms O = 9.03 x 1024 atoms O 1 mol NO3 1 molecule NO3

PRACTICE:

2. How many molecules are there in 2.0 moles of FeSO4? _________

3. How many moles are in 7.5 x 1019 molecules of ZnSO4? ________

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4. How many oxygen atoms are there 0.75 moles of CO2? _________

5. How many moles is 1.50 x 1023 molecules of NH3?

6. How many atoms are in 1.75 mol CHCl3?

7. What is molar mass and from where do you find the information needed for this?

Find the molar mass of the following elements:

a) K = ________ b) Ag = ________ c) Cl = _________

6. How would you find the molar mass of a compound like NaCl?

Find the molar mass of the following compounds:

a) KCl = ________

b) H2CO3 = ________

c) CaSO4 = __________

d) Ca(NO3) 2 = ________

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10.2 Mole-Mass and Mole-Volume Relationships


- STP

- molar volume

Converting Moles to Mass Converting Mass to Moles

1 Mole = molar mass of element or compound (in grams)

therefore

1 mol or molar mass (g) molar mass (g) 1 mol

Example 1: How many moles are in 53.2g of oxygen (O2)?

1) Find the molar mass of oxygen using the periodic table. Since there are 2 atoms of oxygen in O2, you must multiply 16 g oxygen by 2: 1 mol = 32 g oxygen

2) Use the conversion factor to set up your problem. State the given first and multiply by the conversion factor so that the units cancel out on the bottom, and the unit you want it on top:

53.2g O x 1 mol O = 1.66 moles oxygen 32 g O

(moles mass)

If you are given moles and you want to find mass then you multiply by the molar mass of the element or compound.

(mass moles)

If you are given mass and want to find the moles you divide by the molar mass of the element or compound.

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Example 2: What is the mass of 12.3 moles of Al2O3?

1) Find the molar mass of Al2O3 using the periodic table: Al2 = 27g x 2 = 54gO3 = 16g x 3 = 48gMass Al2O3 = 102g so 1 mol = 102g Al2O3

2) Use the conversion factor to set up your problem.

12.3 mol Al2O3 x 102g Al2O3 = 1,254.6g Al2O3

1 mol Al2O3

PRACTICE:1. How many moles are there in 25.0g of NaCl? __________

2. How many moles are there in 100.0g of KMnO4? __________

3. How many grams are in 0.25 moles of KCl? __________

4. How many grams are in 0.50 moles of H2SO4? _________

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Converting Moles to Volume Converting Volume to Moles

1 Mole = 22.4L of gas at STP

therefore

1 mol or 22.4 L gas at STP 22.4 L gas at STP 1 mol

PRACTICE:5. What is the volume of 0.60 moles of SO2 gas at STP?

6. What is the volume of 3.20 x 103 mol CO2 at STP?

7. How many moles are in of 13.70 L of N2 at STP?

8. At STP, how many moles are in of 1.25 L of He?

(moles volume)

If you are given moles and you want to find volume then you multiply by 22.4 L.

(volume moles)

If you are given volume and want to find the moles you divide by the 22.4 L.

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Density = mass therefore Density of gas at STP = molar mass (g) volume 22.4 L

Example: What is the mass of a compound found to be 1.964g/L at STP?

1) Plug in your given into the formula:

Density = molar mass (g) 1.964g/L = molar mass (g) 22.4 L 22.4 L

2) Solve for the mass by multiplying both sides by 22.4L

(1.964g/L) (22.4L) = 44.0g

PRACTICE:9. What is the mass of a gas found to be 3.58g/L at STP?

10. What is the density of krypton gas at STP?

11. What is the mass of a gas found to be 62.3g/L at STP?

12. What is the density of CO2 gas at STP?

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STEPS FOR SOLVING MOLE PROBLEMS (always go to the MOLE first)

1) What is given?2) What is the unknown? 3) Is it a 1-step or 2-step problem?

a. 1-step = mol molar mass mol particles mol volume

b. 2-step = molar mass particles molar mass volume particles volume

4) If it’s a 1-step problem, write the given and solve using the conversion factor.5) If it’s a 2-step problem, write the given, solve for the mol, then solve for the unknown.

Use the following MOLE ROAD MAP to help you solve mole problems:

*** PRACTICE THE PROBLEMS ON CLASS HANDOUTS ***

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10.3 Percent Composition and Chemical Formulas


- percent composition

- empirical formula

1. What formula can you use to find the percent composition from mass data?

2. What formula can you use to find the percent composition from a chemical formula?

PRACTICE: 3. A 13.60g sample of a compound made up of magnesium and oxygen has 5.40g of oxygen. What is the percent composition of this compound?

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4. A compound is formed when 9.03g Mg combines completely with 3.48g N. What is the percent composition of the compound?

5. A 14.2g sample of mercury (II) oxide has 13.2g of Hg. What is the percent composition of this compound?

HYDRATES:To find the percent of water in a hydrate use the following equation:

% water = mass of water x 100 mass of the entire hydrate

(including the water)

6. What percentage of water is found in CuSO4 5H2O?

7. A 2.5g sample of a hydrate of Ca(NO3)2 was heated, and 1.7g of the anhydrous salt remained. What percentage of water was in the hydrate?

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EMPIRICAL FORMULAS

To find an empirical formula given percentages:1) Change each percentage into grams (notice all percentages add up to 100)2) Convert grams to moles for each element3) Divide each number of moles by the smaller number of moles to get the

smallest whole-number ratio4) If the ratio is not in whole-number form, multiply by the smallest number

possible to get a whole-number ratio. These are the subscripts for the formula

Ex: A compound has 25.9% nitrogen and 74.1% oxygen. What is the empirical formula of the compound?

25.9% N 25.9g N x 1 mol N = 1.85 mol N 14 g N

74.1% O 74.1g O x 1mol O = 4.63 mol O 16 g O

1.85 mol N = 1 mol N;4.63 mol O = 2.5 mol O 1.85 1.85

Since 1:2.5 is not a whole-number ratio, multiply by 2 to get whole numbers:

1 mol N x 2 = 2 mol N2.5 mol O x 2 = 5 mol O

Empirical formula = N2O5

To find an empirical formula given mass:1) Follow steps 2-4 in procedure to find the empirical formula given percentage

PRACTICE:8. When iron oxide Fe2O3 reacts with 18.94g of aluminum metal, iron is produced along with 35.74g of aluminum oxide. What is the empirical formula for aluminum oxide?

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9. Find the empirical formula for the following for a compound that is 75% carbon and 25% hydrogen

10. When 20.16g of magnesium oxide reacts with carbon, carbon monoxide and 12.16g of magnesium are produced. What is the empirical formula of magnesium oxide?

11. Calculate the empirical formula for a compound that is 94.1% oxygen and 5.9% hydrogen.

12. Calculate the empirical formula for a compound that is 67.6% mercury, 10.8% sulfur, and 21.6% oxygen.

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MOLECULAR FORMULAS

13. What is a molecular formula?

To find a molecular formula:1) You will be given the molar mass of the compound2) Find the empirical formula if it is not given3) Find the empirical formula mass (efm) of the compound4) Divide the given molar mass by the efm to get a whole number (n)5) Multiply this whole number (n) by each formula subscript to get the

molecular formula.

Ex: What is the molecular formula for a compound whose molar mass is 34g and whose empirical formula is HO?

Empirical formula mass = 17g (molar mass H = 1g + molar mass O = 16g)

molar mass = 34g = 2 ( this is n, so multiply it by the subscripts)empirical formula mass 17g

(HO)2 = H2O2

PRACTICE:

14. Find the molecular formula of a compound that is 62g/mol and whose empirical formula is CH3O.

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