Question

1. how many moles of pb are in 4.71 x 10^21 pb atoms?

a) 0.00782
b) 6.022 x 10^23
c) 2.84 x 10^45
d) 207.2
e) none of the above

1. one mole of oxygen gas has a mass of ______ g.

a) 32.0
b) 8
c) 6.022 x 10^23
d) 16.0
e) none of the above

1. you have 10.0 g each of na, c, pb, cu and ne. which contains the smallest number of moles?

a) pb
b) c
c) cu
d) na
e) ne

1. how many moles are there in 17.5 grams of sodium?

a) 0.761
b) 1.31
c) 22.99
d) 1.05 x 10^25
e) none of the above

1. how many hydrogen atoms are in 35.0 grams of hydrogen gas?

a) 2.12 x 10^25
b) 2.09 x 10^25
c) 4.25 x 10^25
d) 1.05 x 10^25
e) none of the above

1. what is the mass of 3.09 x 10^24 atoms of sulfur in grams?

a) 0.160
b) 9.91 x 10^25
c) 165
d) 9.64 x 10^22
e) none of the above

1. the mass of one mole of carbon dioxide is ______ g.

a) 44.01
b) 32.00
c) 28.01
d) 384.4
e) none of the above

Explanation:

Step1: Recall Avogadro's number

Avogadro's number $N_A=6.022\times 10^{23}\text{ atoms/mol}$. To find moles from atoms, use the formula $n=\frac{N}{N_A}$, where $n$ is the number of moles and $N$ is the number of atoms.
For question 15: Given $N = 4.71\times 10^{21}$ Pb atoms. Then $n=\frac{4.71\times 10^{21}}{6.022\times 10^{23}}= 0.00782$ mol.

Step2: Recall molar - mass concept

The molar mass of a substance is the mass of one - mole of that substance.
For question 16: Oxygen gas is $O_2$. The molar mass of $O$ is $16.0$ g/mol, so the molar mass of $O_2$ is $2\times16.0 = 32.0$ g/mol.

Step3: Calculate moles from mass

The formula to calculate moles from mass is $n=\frac{m}{M}$, where $m$ is the mass and $M$ is the molar mass.
For question 17:
The molar mass of Na is $22.99$ g/mol, $n_{Na}=\frac{10.0}{22.99}\approx0.435$ mol;
The molar mass of C is $12.01$ g/mol, $n_{C}=\frac{10.0}{12.01}\approx0.833$ mol;
The molar mass of Pb is $207.2$ g/mol, $n_{Pb}=\frac{10.0}{207.2}\approx0.0483$ mol;
The molar mass of Cu is $63.55$ g/mol, $n_{Cu}=\frac{10.0}{63.55}\approx0.157$ mol;
The molar mass of Ne is $20.18$ g/mol, $n_{Ne}=\frac{10.0}{20.18}\approx0.495$ mol. Pb has the smallest number of moles.
For question 18: Given $m = 17.5$ g of Na and $M_{Na}=22.99$ g/mol. Then $n=\frac{17.5}{22.99}\approx0.761$ mol.
For question 19: Hydrogen gas is $H_2$. The molar mass of $H_2$ is $2.016$ g/mol. First, find the number of moles of $H_2$, $n=\frac{35.0}{2.016}\approx17.36$ mol. Since 1 mole of $H_2$ contains 2 moles of H atoms, the number of moles of H atoms is $n_H = 2\times17.36 = 34.72$ mol. Then the number of H atoms is $N=n_H\times N_A=34.72\times6.022\times 10^{23}=2.09\times 10^{25}$ atoms.
For question 20: First, find the number of moles of S atoms. Given $N = 3.09\times 10^{24}$ atoms of S, $n=\frac{3.09\times 10^{24}}{6.022\times 10^{23}}\approx5.13$ mol. The molar mass of S is $32.06$ g/mol. Then the mass $m=n\times M = 5.13\times32.06\approx165$ g.
For question 21: The formula of carbon - dioxide is $CO_2$. The molar mass of C is $12.01$ g/mol and the molar mass of O is $16.0$ g/mol. So the molar mass of $CO_2$ is $12.01+2\times16.0 = 44.01$ g/mol.

Answer:

1. A. 0.00782
2. A. 32.0
3. A. Pb
4. A. 0.761
5. B. $2.09\times 10^{25}$
6. C. 165
7. A. 44.01