## Chemistry 8 Lesson 20: Mass density of gases

## 1. Summary of theory

### 1.1. How do you know if gas A is heavy or light, gas B?

+ dA/B > 1 Gas A is heavier than gas B

+ dA/B = 1 Gas A is as heavy as gas B

+ dA/B < 1 Gas A is lighter than gas B

### 1.2. How can we know whether gas A is heavier or lighter than air?

– d_{A/KK} : Mass ratio of gas A to air

– USA_{A} : Molar mass of gas A.

– USA_{KK} : Molar mass of air.

In biology, we know that air is a mixture of many gases, two of which are N . gas_{2} accounts for about 80% and O . gas_{2} account for about 20%. Therefore, the mass of “moles of air” is the mass of 0.8 moles of nitrogen gas + the mass of 0.2 moles of oxygen.

So MOTHER_{kk} = (0.8 x 28g) + ( 0.2 x 32g) ≈ 29 grams

**– Eg:** CO . gas_{2} How many times heavier or lighter than air?

### 1.3. summary

## 2. Illustrated exercise

### 2.1. Form 1: Calculate the cubic rate

**Lesson 1: **Which of the following gases is the heaviest?

A. ONLY_{4 }

B. CO_{2 }

CN_{2 }

D. FAMILY_{2}

**Solution guide**

d_{CO2/kk} = 1.5 (1)

d_{CH4/kk} = 0.55

d_{N2/kk} = 0.96

d_{H2/kk} = 0.07

I see d_{CO2/kk} the largest so CO_{2} heaviest

**Lesson 2: **Calculate the vapor density of a mixture of Y with equal mass of gas C_{2}H_{4} (ethylene), FEMALE_{2} and CO versus H_{2}.

**Solution guide**

Mixture Y consists of C_{2}H_{4}, female_{2} and CO have the same mass, on the other hand we find that the molar masses of these three gases are equal and equal to 28, so the number of moles of each gas in Y are equal.

d_{MY/H2} = 28 : 2 = 14

### 2.2. Form 2: Calculate the molar mass

**Lesson 1: **Calculate the molar mass of gas A, given that the density of gas A with respect to oxygen gas is 2

**Solution guide**

Apply the formula to calculate the density: dA/O_{2} = US_{A}/MOTHER_{O2} → US_{A} = 32. 2 = 64

**Lesson 2: **Calculate the molar mass of gas B, given that B is 1.52 times heavier than air

**Solution guide**

Apply the formula to calculate the density dB/kk = M_{REMOVE}/MOTHER_{kk} → MB = 29.1.52 = 44.08

### 2.3. Form 3: Compound exercises

The gas mixture consists of 11.2 liters of N2 gas and 33.6 liters of O2 at the location

a) Calculate the mass of the gas mixture.

b) Calculate the percent by mass of each gas in the mixture.

c) Is this mixture of gases heavier or lighter than air?

**Solution guide**

a. nN_{2 }= 11.2 22.4 = 0.5 mol

nO_{2} = 33.6 : 22.4 = 1.5 mol

→ mN_{2 }= 0.5 . (14. 2) = 14 g

mO_{2 }= 1.5. 16. 2 = 48 g

So total mass of gas mixture = 14 + 48 = 62 g

b. % by mass of N_{2 }= (14:62). 100 = 22.58%.

% by mass of O_{2} = 100% – 22.58% = 77.42%

c. Apply the formula: M = \({M_{hh}} = \frac{{0.5.(14.2) + 1.5.(16.2)}}{{0.5 + 1.5}} = 31 \) → dhh/kk = 31 : 29 = 1.06

## 3. Practice

### 3.1. Essay exercises

**Question 1:** Vapor mass ratio of gas X to methane CH_{4} equals 4. Find the molar mass of gas X.

**Verse 2: **Gas mixture consisting of 11.2 liters of N . gas_{2} and 33.6 liters of O . gas_{2} in dktc.

a) Calculate the mass of the gas mixture.

b) Calculate the percent by mass of each gas in the mixture.

c) Is this mixture of gases heavier or lighter than air.

**Question 3:** A mixture X of H_{2} to enter_{2} (no reaction occurs) has a density relative to air of 0.3276.

a) Calculate the average molar mass of the mixture.

b) Calculate the percent composition in moles of each gas in the mixture (in two different ways).

**Verse 4: **16 g of gas A has a vapor density for methane ONLY_{4} equals 4.

a) Calculate the molar mass of gas A.

b) Calculate the volume of gas A at dtc.

**Sentence ****5****:** Let gas A have a vapor density for methane gas ONLY_{4} equals 2.75. Find the molar mass of gas B, given that the mass ratio of gas B to gas A is 1.4545.

### 3.2. Multiple choice exercises

**Question 1:** Of the following gases, the number of gases lighter than air is: CO_{2}, H_{2}O, FEMALE_{2}, H_{2}, SO_{2}

A. 5

B. 4

C. 3

D. 2

**Verse 2:** Of the following gases, the number of gases heavier than air is: CO_{2}, H_{2}O, FEMALE_{2}, H_{2}, SO_{2}, female_{2}O

A. 1

B. 3

C. 4

D. 5

**Question 3:** SO gas_{2} How many times heavier or lighter than air?

A. 2.2 times heavier than air

B. 3 times lighter than air

C. 2.4 times heavier than air

D. 2 times lighter than air

**Verse 4:** Feminine gas_{2} heavier or lighter than O . gas_{2}

A. WOMEN_{2} heavier than O_{2} 1.75 times

B. O_{2} heavier than WOMEN_{2} 1.75 times

CN_{2} = O_{2}

D. Not qualified to conclude

**Question 5:** For CO_{2}, H_{2}O, FEMALE_{2}, H_{2}, SO_{2}, female_{2}O, ONLY_{4}, NS_{3}. The gas that can be obtained by standing the flask is

A. CO_{2}, ONLY_{4}, NS_{3}

B. CO_{2}, H_{2}O, ONLY_{4}, NS_{3}

C. CO_{2}, SO_{2}, female_{2}O

D. WOMEN_{2}, H_{2}, SO_{2}, female_{2}O, ONLY_{4}, NS_{3}

## 4. Conclusion

After the lesson need:

- How to determine the density of gas A relative to gas B
- Know how to determine the density of a gas relative to air.

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