Chemical Composition Review Mole Calculations Percent ... Chemical... · atoms versus the number of...

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Chemical Composition

Review

Mole Calculations

Percent Composition


QUESTION

Suppose you work in a hardware store and a customer wants to

purchase 500 nails. If the average mass of a nail is 20 grams, how

many kilograms of nails would you weigh out?

1. 10 kg

2. 103 kg

3. 102 kg

4. 25 kg


ANSWER

Choice #1 is the correct answer. This problem illustrates the

concept of counting by weighing. Rather than laboriously

counting out 500 nails we use the average mass of a nail to

compute the mass of 500 nails and simply weigh out the correct

mass.

20 g 1 kg500 nails 10 kg

1 nail 1000 g


QUESTION

A pile of marbles weigh 394.80 g. 10 marbles weigh 37.60 g.

How many marbles are in the pile?

1. 11

2. 40

3. 105

4. 223


ANSWER

Choice #3 is the correct answer.

37.60 gAvg. Mass of 1 Marble = = 3.76 g / marble

10 marbles

394.80 g = 105 marbles3.76 g


QUESTION

Atomic masses on the periodic table are not reported as whole numbers. What is the chief reason for this?

1. Atoms are so small that even modern techniques have a limited accuracy, which keeps the value from being a whole number.

2. Atomic mass scales are based on hydrogen 1.01, and since this atom does not have a whole number mass the comparisons will not be able to have whole number masses also.

3. Isotopic abundance is averaged to provide the mass of atoms.

4. Atomic mass is based on a variable scale.


ANSWER

Choice #3 properly explains that the masses of elements are

based on the weighted average of the isotopes for each element.


QUESTION

Since it is extremely difficult to “see” atoms without elaborate

equipment, chemists use the atomic mass scale to determine the

number of atoms in a sample. For example, calculate the number

of aluminum atoms in a piece of aluminum foil that weighs 2.58

× 108 amu.

1. 9.56 x 106

2. 2.58 x 108

3. 2.58 x 106

4. 1.33 x 1020


ANSWER

Choice #1 provides the correct answer.

8 61 atom Al2.58 10 amu 9.56 10 atoms Al

26.98 amu


QUESTION

Diamond is an allotrope of carbon. If 1.0 carat has a mass of 0.20

grams, how many carbon atoms are in a 3.0 carat diamond?

1. 6.0 x 1015

2. 7.2 x 1010

3. 3.0 x 1026

4. 3.0 x 1022


ANSWER

Choice #4 correctly gives the atomic make up of the 3.0 carat

diamond.

24

22

0.20 g C 1 amu C 1 atom C3.0 c

1.0 c 12.01 amu C1.66 10 g C

3.0 10 atoms C


QUESTION

Calculate the number of iron atoms in a 4.48 mole sample of iron.

1. 7.44 × 10–24

2. 1.34 × 1023

3. 12.5

4. 2.70 × 1024


ANSWER


23246.022 10 atoms Fe

4.48 mol Fe 2.70 10 atoms Fe1 mol Fe


QUESTION

The metal gallium (Ga) has most of the properties you would

expect in a metal, but it is so soft it will melt in your hand. If you

had 20.0 grams of solid gallium in your palm and watched it turn

into liquid, how many moles of liquid gallium would you soon

have?

1. 20.0 moles

2. 0.287 moles

3. 4.32 moles

4. 3.6 1023 moles


ANSWER

Choice #2 represents the mole equivalent for 20.0 grams of

gallium.

1 mole Ga20.0 g Ga 0.287 moles Ga

69.7 g Ga


QUESTION

Which of the following is closest to the average mass of one atom

of copper?

1. 63.55 g

2. 58.93 g

3. 65.38 g

4. 1.055 x 10–22 g


ANSWER

Choice #4 is the correct answer. The mass of one atom of copper

is going to be extremely small, so only Choice #4 makes sense.

The calculated solution is:

22

23

1 mol Cu 63.55 g Cu1 Cu atom = 1.055 10 g Cu

1 mol Cu6.022 10 Cu atoms


QUESTION

A sample of 26.98 grams of Al has the same number of atoms as

_______ grams of Au.

1. 26.98 g

2. 13.49 g

3. 197.0 g

4. 256.5 g


ANSWER

Choice #3 gives the mass of 1.0 mole of gold. The 26.98 g of Al

represents 1.0 mole of Al. Thus both samples contain Avogadro’s

number, 6.022 × 1023 atoms.


QUESTION

Which silver ring would be worth more?

1. A ring that weighs 10.0 grams.

2. A ring that consists of 1.0 × 1022 atoms of Ag.


ANSWER

Choice #1 is the correct answer. The ring that weighs 10.0 g

would be worth more. We can tell by using dimensional analysis

to figure out how much the 1.0 × 1022 atoms of Ag would weigh:

22

23

1 mole Ag 107.9 g Ag1.0 10 atoms Ag 1.8 g Ag

1 mole Ag6.022 x 10 atoms Ag


QUESTION

Which of the following 100.0 g samples contains the greatest

number of atoms?

1. Magnesium

2. Zinc

3. Silver


ANSWER

Choice #1 is the correct answer. Magnesium has the smallest

average atomic mass (24.31 amu). Since magnesium is lighter

than zinc and silver, it will take more atoms to reach 100.0 g.


QUESTION

You have two beakers on your lab table. Beaker #1 contains 32.07 g of sulfur (S) and Beaker #2 contains 74.92 g of arsenic (As). Which beaker contains the greatest number of atoms? Choose the best answer.

1. Beaker #1 because one sulfur atom weighs less than one arsenic atom so you need more of the sulfur atoms to fill the beaker.

2. Beaker #2 because arsenic has a greater mass than sulfur.

3. Beaker #2 because it contains more moles of arsenic atoms versus the number of moles of sulfur atoms in Beaker #1.

4. Beakers #1 and #2 contain the same number of atoms because there is one mole in each.


ANSWER

Choice #4 is the correct answer. They each contain one mole of

atoms, which is equivalent to Avogadro’s number.


QUESTION

One mole of carbon is equivalent to

1. 12.01 atomic mass units.

2. the same mass of atoms that are contained in one mole of

calcium.

3. the same number of atoms that are contained in one mole

of calcium.

4. 6.022 × 1023 grams of carbon.


ANSWER



QUESTION

What is the molar mass of nickel(II) carbonate?

1. 118.7 g/mol

2. 134.7 g/mol

3. 178.71 g/mol

4. 296.09 g/mol


ANSWER

Choice #1 is the correct answer. The formula for nickel(II)

carbonate is NiCO3. Therefore the molar mass is: 58.69 + 12.01

+ 3(16.00) = 118.7 g/mol.


QUESTION

The molar mass of ammonium phosphate is:

1. 131.05 g/mol

2. 144.06 g/mol

3. 149.10 g/mol

4. 302.95 g/mol


ANSWER

Choice #3 is the correct answer. The formula for ammonium

phosphate is (NH4)3PO4. Therefore the molar mass is: 3(14.01) +

12(1.008) + 30.97 + 4(16.00) = 149.10 g/mol.


QUESTION

Consider equal mole samples of N2O, Al(NO3)3, and KCN. Rank

these from least to most number of nitrogen atoms in each

sample.

1. KCN, Al(NO3)3, N2O

2. Al(NO3)3, N2O, KCN

3. KCN, N2O, Al(NO3)3

4. Al(NO3)3, KCN, N2O


ANSWER

Choice #3 is the correct answer. Assume you have one mole of

each sample. This means you have 1 mole of N in KCN, 2 moles

of N in N2O and 3 moles of N in Al(NO3)3. Moles are directly

related through Avogadro’s number.


QUESTION

You have 27.0 g of an unknown substance A and 75.0 g of

chlorine gas. Substance A contains 1.5 times as many molecules

as the chlorine gas. What is the identity of A?

1. HClO

2. NO2

3. BF3

4. NH3


ANSWER

Choice #4 is the correct answer. To identify A, you need to know

the molar mass (# grams/# moles). The number of grams of A is

already given. To find the moles of A:

Therefore the molar mass of A is 17 g/mol (27.0 g/1.6 mol),

which is NH3.

2

2

2 2

1 mol Cl 1.5 mol A75.0 g Cl = 1.6 mol A

70.9 g Cl 1 mol Cl


QUESTION

How many grams of fluorine are contained in one molecule of

boron trifluoride?

1. 3.155 × 10–23 g

2. 9.465 × 10–23 g

3. 6.022 × 1023 g

4. 3.433 × 1025 g


ANSWER


233

3

23

3 atoms F 1 mol F 19.00 g F1 molecule BF

1 molecule BF 1 mol F6.022 10 atoms F

= 9.465 10 g F


QUESTION

Morphine, derived from opium plants, has the potential for use

and abuse. It’s formula is C17H19NO3. What percent, by mass, is

the carbon in this compound?

1. 12.0 %

2. 54.8 %

3. 67.9 %

4. 71.6 %


ANSWER

Choice #4 is correct. First determine the molar mass of the

compound, then divide that into the total mass of carbon present.

Finally multiply that by 100 to convert from a fraction into a

percentage.

17 12.01 = 0.716 100 = 71.6% C

17 12.01 + 19 1.008 + 14.01 + 3 16.00


QUESTION

What is the mass percent of potassium in potassium nitride?

1. 89.33%

2. 48.19%

3. 38.67%

4. 29.78%


ANSWER

Choice #1 is the correct answer. The formula for potassium

nitride is K3N.

3 39.10 = 0.8933 100 = 89.33% K

3 39.10 + 14.01


QUESTION

What is the percent mass of phosphorus in calcium phosphide?

1. 19.97%

2. 34.00%

3. 43.59%

4. 53.68%


ANSWER

Choice #2 is the correct answer. The formula for calcium

phosphide is Ca3P2.

2 30.97 = 0.3400 100 = 34.00% P

3 40.08 + 2 30.97


QUESTION

A gas consisting of 50.0% sulfur and 50.0% oxygen by mass has

what empirical formula?

1. SO

2. SO2

3. S2O

4. SO4


ANSWER

Choice #2 is the smallest whole number ratio of the atoms that make up this compound. When presented with % composition data, we start out by assuming we have 100 g of that compound so that the %’s are masses in grams. We then convert the masses into moles using atomic masses:

We then divide by the smallest # of moles: 3.13/1.56 = 2

Therefore # moles O/# moles S = 2/1 …SO2

1 mole S50.0 g S 1.56 mole S

32.06 g S

1 mole O50.0 g O 3.13 mole O

16.00 g O


QUESTION

A hydrocarbon (binary compound containing carbon and

hydrogen) has a percent composition of 92% carbon and

8% hydrogen and a molar mass of about 26 g/mol. Determine the

molecular formula of this compound.

1. CH4

2. C2H4

3. C2H2

4. CH2


ANSWER

Choice #3 is the only formula listed with a molar mass of 26 g/mol. To see how the formula is worked out, we convert the %’s to grams and divide by atomic masses:

7.7 rounds off to 8 so C/H is 1/1 in the compound. The empirical formula is CH, but this has a molecular weight of 13. 26/13 = 2, so we multiply the formula CH by 2 to give C2H2.

1 mole C92 g C 7.7 mole C

12.01 g C

1 mole H8 g H x 8 mole H

1.008 g H


QUESTION

Vinegar contains carbon, hydrogen, and oxygen with percent

masses of 40.01% C and 6.70% H, respectively. If the molar

mass of vinegar is about 60 g/mol, what is its molecular formula?

1. CH2O

2. C3H8O

3. C2H20O

4. C2H4O2


ANSWER

Choice #4 is the correct answer. The empirical formula is CH2O

with a molar mass of about 30 g/mol. 60/30 = 2, so we multiply

the formula CH2O by 2 to get C2H4O2.


QUESTION

A compound consists of silicon and 8.23% by mass of hydrogen.

The molar mass of the compound is approximately 122 g/mol.

What is the molecular formula of this compound?

1. Si4H10

2. SiH2

3. SiH3

4. Si2H5


ANSWER

Choice #1 is the correct answer. The empirical formula is Si2H5

with a molar mass of about 61 g/mol. 122/61 = 2, so we multiply

the formula Si2H5 by 2 to get Si4H10.

Chemical Composition Review Mole Calculations Percent ... Chemical... · atoms versus the number of...

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