Q: Which equation correctly represents the calculation of average atomic mass for an element with multiple isotopes?

$\text{Average Atomic Mass} = \sum (\text{fractional abundance} \times \text{isotopic mass})$

Q: A sample of element X consists of 62.60% of an isotope with mass 186.956 amu and 37.40% of an isotope with mass 184.953 amu. What is the average atomic mass of element X?

$(0.6260 \times 186.956) + (0.3740 \times 184.953) = 186.22$ amu

Q: Which statement best explains why the atomic mass of carbon listed on the periodic table is not a whole number?

It is a weighted average of the masses of all naturally occurring isotopes of carbon.

Q: Which of the following best describes the concept of 'counting by weighing' in chemical stoichiometry?

Using the average mass of identical objects to determine the total number present by dividing the total mass by the average mass.

Q: A pile of marbles weighs 394.80 g. If 10 marbles weigh 15.20 g, how many marbles are in the pile?

$\frac{394.80\ \text{g}}{1.52\ \text{g/marble}} = 259.7$ marbles

Q: What is Avogadro's number and what does it represent?

$6.022 \times 10^{23}$; the number of units (atoms, molecules, etc.) in one mole of a substance.

Q: If the molar mass of silver (Ag) is $107.87$ g/mol, how many moles are present in a $125$ g sample of silver?

$\frac{125\ \text{g}}{107.87\ \text{g/mol}} = 1.16$ mol

Q: How many iron atoms are present in a $4.48$ mole sample of iron?

$4.48 \times 6.022 \times 10^{23} = 2.70 \times 10^{24}$ atoms

Q: Which of the following is the correct definition of a mole in chemistry?

A mole is the amount of substance that contains $6.022 \times 10^{23}$ entities (atoms, molecules, etc.).

Q: Which step is NOT part of the recommended problem-solving strategy in chemical calculations?

Estimate the answer before reading the problem.

Question 1

Which equation correctly represents the calculation of average atomic mass for an element with multiple isotopes?

Accepted Answer

$	ext{Average Atomic Mass} = \sum (	ext{fractional abundance} 	imes 	ext{isotopic mass})$

Question 2

Carbon has three naturally occurring isotopes: $^{12}	ext{C}$ (98.89% at 12.000 amu), $^{13}	ext{C}$ (1.11% at 13.0034 amu), and $^{14}	ext{C}$ (<0.01% at 14.0032 amu). What is the average atomic mass of carbon?

Accepted Answer

$12.011$ amu

Question 3

A sample of element X consists of 62.60% of an isotope with mass 186.956 amu and 37.40% of an isotope with mass 184.953 amu. What is the average atomic mass of element X?

Accepted Answer

$(0.6260 	imes 186.956) + (0.3740 	imes 184.953) = 186.22$ amu

Question 4

Which statement best explains why the atomic mass of carbon listed on the periodic table is not a whole number?

Accepted Answer

It is a weighted average of the masses of all naturally occurring isotopes of carbon.

Question 5

Which of the following best describes the concept of 'counting by weighing' in chemical stoichiometry?

Accepted Answer

Using the average mass of identical objects to determine the total number present by dividing the total mass by the average mass.

Question 6

A pile of marbles weighs 394.80 g. If 10 marbles weigh 15.20 g, how many marbles are in the pile?

Accepted Answer

$\frac{394.80\ 	ext{g}}{1.52\ 	ext{g/marble}} = 259.7$ marbles

Question 7

What is Avogadro's number and what does it represent?

Accepted Answer

$6.022 	imes 10^{23}$; the number of units (atoms, molecules, etc.) in one mole of a substance.

Question 8

If the molar mass of silver (Ag) is $107.87$ g/mol, how many moles are present in a $125$ g sample of silver?

Accepted Answer

$\frac{125\ 	ext{g}}{107.87\ 	ext{g/mol}} = 1.16$ mol

Question 9

How many iron atoms are present in a $4.48$ mole sample of iron?

Accepted Answer

$4.48 	imes 6.022 	imes 10^{23} = 2.70 	imes 10^{24}$ atoms

Question 10

A sample contains $6.00 	imes 10^{23}$ atoms of an element with atomic mass $195.08$ amu. What is the mass in grams of the sample?

Accepted Answer

$195.08$ g

Question 11

Which of the following is the correct definition of a mole in chemistry?

Accepted Answer

A mole is the amount of substance that contains $6.022 	imes 10^{23}$ entities (atoms, molecules, etc.).

Question 12

Which step is NOT part of the recommended problem-solving strategy in chemical calculations?

Accepted Answer

Estimate the answer before reading the problem.

Question 13

Why is it valid to use the average atomic mass for stoichiometric calculations, even though no single atom has that mass?

Accepted Answer

Because large samples behave as though all atoms are identical and have the average mass.

Question 14

Which of the following is true about isotopes?

Accepted Answer

Isotopes of an element have the same number of protons but different numbers of neutrons.

Question 15

If you have $2.00$ moles of carbon atoms, what is the total mass in grams?

Accepted Answer

$2.00 	imes 12.01 = 24.02$ g

Question 16

Which of the following is the correct relationship between atomic mass units (amu) and grams?

Accepted Answer

$1$ amu $ \approx 1.66 \times 10^{-24} $ g

Question 17

Which of the following is a real-world application of stoichiometry?

Accepted Answer

Determining the amount of reactants needed to produce a specific amount of product in a chemical reaction.

Question 18

A chemist needs $3.00$ moles of sodium for a reaction. If the molar mass of sodium is $22.99$ g/mol, what mass of sodium should be weighed out?

Accepted Answer

$3.00 	imes 22.99 = 68.97$ g

chapter 2 p2

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