# Topic 7: Stoichiometry Part 1

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• 1.

### How many molecules of water are there in 36.0 grams of the substance?

• A.

6.0 X 10^23

• B.

1.2 X 10^24

• C.

2.8X 10^23

• D.

3.2 X 10^21

• E.

1.8 X 10^24

B. 1.2 X 10^24
Explanation
The molar mass of water (H2O) is approximately 18 g/mol. To find the number of molecules, we need to convert grams to moles using the molar mass.

36.0 g / 18 g/mol = 2 moles of water

Next, we use Avogadro's number (6.022 x 10^23 molecules/mol) to convert moles to molecules:

2 moles x (6.022 x 10^23 molecules/mol) = 1.2 x 10^24 molecules of water

Therefore, there are 1.2 x 10^24 molecules of water in 36.0 grams of the substance.

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• 2.

### Molar Mass of glucose (C6H12O6) in grams per mole?

• A.

340.0

• B.

180.0

• C.

110.0

• D.

78.0

• E.

44.0

B. 180.0
Explanation
The molar mass of glucose (C6H12O6) can be calculated by adding up the atomic masses of all the atoms in one molecule of glucose:
Carbon (C) has an atomic mass of approximately 12 g/mol.
Hydrogen (H) has an atomic mass of approximately 1 g/mol.
Oxygen (O) has an atomic mass of approximately 16 g/mol.
Now, calculate the molar mass of glucose (C6H12O6):
(6 × Atomic Mass of Carbon) + (12 × Atomic Mass of Hydrogen) + (6 × Atomic Mass of Oxygen) = 72 g/mol + 12 g/mol + 96 g/mol = 180 g/mol

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• 3.

### Molar Mass of glucose (CO2) in grams per mole?

• A.

340.0

• B.

180.0

• C.

110.0

• D.

78.0

• E.

44.0

E. 44.0
Explanation
The molar mass of glucose (C6H12O6) is 180.0 grams per mole. The molar mass of carbon dioxide (CO2) is 44.0 grams per mole. Since the question asks for the molar mass of glucose (CO2), it is incorrect and the given answer of 44.0 is incorrect.

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• 4.

### What is the molar mass of H2  i in grams per mole

• A.

15.0

• B.

2.0

• C.

4.0

• D.

5.2

• E.

3.5

B. 2.0
Explanation
The molar mass of H2 is 2.0 grams per mole because the molar mass of hydrogen (H) is approximately 1.0 grams per mole, and since there are two hydrogen atoms in H2, the molar mass is doubled to 2.0 grams per mole.

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• 5.

### What is the molar mass of NO2  in grams per mole

• A.

16.2

• B.

46.0

• C.

44.0

• D.

13.1

• E.

None of the above

B. 46.0
Explanation
The molar mass of NO2 is 46.0 grams per mole.

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• 6.

### What is the molar mass of benzene C6H6 ?( in grams per mole)

• A.

44.0

• B.

33.1

• C.

57.2

• D.

46.9

• E.

78.0

E. 78.0
Explanation
The molar mass of benzene (C6H6) is 78.0 grams per mole. This can be calculated by adding up the atomic masses of each element in the chemical formula. The atomic mass of carbon (C) is 12.01 grams per mole, and the atomic mass of hydrogen (H) is 1.01 grams per mole. Multiplying the atomic mass of carbon by 6 (since there are 6 carbon atoms in benzene) and the atomic mass of hydrogen by 6 (since there are 6 hydrogen atoms in benzene), and then adding these values together gives a total molar mass of 78.0 grams per mole.

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• 7.

### 1.8 X 10^24 atoms of  sodium contains approximately how many grams of this substance?

• A.

83

• B.

69

• C.

17

• D.

11

• E.

23

B. 69
Explanation
The correct answer is 69 because the question asks for the number of grams of sodium in 1.8 X 10^24 atoms. The atomic mass of sodium is approximately 23 grams per mole. Therefore, to find the number of grams in 1.8 X 10^24 atoms, we divide 1.8 X 10^24 by Avogadro's number (6.022 X 10^23) to get approximately 2.99 moles. Finally, we multiply the number of moles by the atomic mass of sodium to get approximately 69 grams.

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• 8.

### 6.02 X 10^23 is known as

• A.

Charles' number

• B.

Irrational number

• C.

• D.

Boyle's law

• E.

None of the above

Explanation
Avogadro's number, 6.02 x 10^23, is a fundamental constant in chemistry and physics. It represents the number of particles (atoms, molecules, ions, etc.) in one mole of a substance. This number is named after Amedeo Avogadro, an Italian scientist who proposed the concept of Avogadro's hypothesis, which states that equal volumes of gases at the same temperature and pressure contain an equal number of particles. Avogadro's number is widely used in various calculations and is essential in understanding the relationship between the macroscopic and microscopic properties of matter.

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• 9.

### What is the molar mass of NH3? ( in grams per mole)

• A.

13

• B.

14

• C.

15

• D.

17

• E.

20

D. 17
Explanation
The molar mass of a compound is the sum of the atomic masses of all the atoms in one mole of the compound. In the case of NH3, nitrogen (N) has an atomic mass of 14 and hydrogen (H) has an atomic mass of 1. Since there are three hydrogen atoms in NH3, the molar mass of NH3 is calculated as (1 x 3) + 14 = 17 grams per mole.

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• 10.

### What is the molar mass of CH4

• A.

16 grams per mole

• B.

20 grams per mole

• C.

24 grams per mole

• D.

30 grams per mole

• E.

14 grams per mole

A. 16 grams per mole
Explanation
The molar mass of CH4 is 16 grams per mole because it consists of one carbon atom with a molar mass of 12 grams per mole and four hydrogen atoms with a molar mass of 1 gram per mole each. Adding up the molar masses of each atom gives a total molar mass of 16 grams per mole.

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