CH.20 Molarity, Molality, Density - CH.28 Partial Pressure

Question 1

Which of the following would affect molality but not molarity?
A. Temperature change
B. Mass of solute
C. Type of solute
D. Volume of solution

Answer 1

Answer: A
✔️ Molarity depends on volume, which changes with temp; molality doesn’t.

Question 2

What is the density of a solution if 500 g of it occupies 250 mL?
A. 2.0 g/mL
B. 0.5 g/mL
C. 1.5 g/mL
D. 2.5 g/mL

Answer 2

Answer: A
✔️ 500 g ÷ 250 mL = 2.0 g/mL

formula: kg or g / mL or L

Question 3

What is the molarity of a mixture made by mixing 10 L of 4 M NaOH with 5 L of 2 M NaOH?
A. 3.2 M
B. 2.5 M
C. 3.0 M
D. 4.5 M

Answer 3

Answer: A
✔️ Use the formula:
total moles/ total liters
M = (M₁V₁ + M₂V₂) / (V₁ + V₂) = (4×10 + 2×5) / (10+5) = (40 + 10)/15 = 3.33 M

Question 4

If you’re told to calculate the moles of solute in 0.5 L of a 6 M solution, what do you do?
A. Divide 6 by 0.5
B. Multiply 6 by 0.5
C. Subtract 0.5 from 6
D. Multiply 6 by 1000

Answer 4

Answer: B
When you get the Molarity (M) and volume just multiply
✔️ Moles = M × L = 6 × 0.5 = 3 mol

Question 5

On the DAT, when water is the solvent, which of the following is usually true?
A. Use molality only
B. Use density instead
C. Molarity and molality give the same value
D. The solute becomes the solvent

Answer 5

C. Molarity and molality give the same value

Question 7

What is the molarity of a solution that contains 2 moles of NaCl in 500 mL of solution?
A. 0.25 M
B. 1.00 M
C. 2.00 M
D. 4.00 M

Answer 7

Answer: D
✔️ Convert mL → L: 500 mL = 0.5 L → M = 2 mol / 0.5 L = 4.0 M

Question 8

If you dilute 1.0 L of 6.0 M H₂SO₄ to 3.0 L, what is the new molarity?
A. 1.0 M
B. 2.0 M
C. 3.0 M
D. 4.0 M

Answer 8

Answer: B
✔️ M₁V₁ = M₂V₂ → (6.0)(1.0) = M₂(3.0) → M₂ = 2.0 M

Question 9

What is the molality of a solution containing 1.5 moles of KCl dissolved in 0.750 kg of water?
A. 1.5 m
B. 0.5 m
C. 2.0 m
D. 3.0 m

Answer 9

Answer: C
✔️ Molality = mol / kg → 1.5 / 0.75 = 2.0 m

Question 10

If you dissolve 0.25 mol of NaOH into 250 g of water, what is the molality?
A. 0.50 m
B. 1.00 m
C. 2.00 m
D. 4.00 m

Answer 10

Answer: B
✔️ 250 g = 0.250 kg → molality = 0.25 / 0.25 = 1.00 m

Question 11

What is the percent composition of Br in CuBr₂?
A. 55.00%
B. 71.55%
C. 28.45%
D. 47.23%

Answer 11

Answer: B
→ Molar mass CuBr₂ = 63.55 + 2(79.90) = 223.35 g/mol
→ Br contribution = 2(79.90) = 159.80 g
→ %Br = (159.80 / 223.35) × 100 = ~71.55%

Question 12

What is the percent composition of O in H₂O₂? (H = 1.01, O = 16.00)
A. 88.79%
B. 94.12%
C. 32.00%
D. 47.06%

Answer 12

Answer: B
→ Molar mass = 2(1.01) + 2(16.00) = 34.02
→ %O = (2×16.00 / 34.02) × 100 = 94.12%

Question 13

What is the empirical formula for a compound that is 40.00% C, 6.72% H, and 53.29% O?
A. C₃H₄O₄
B. CH₂O
C. C₂H₄O
D. C₂H₆O₂

Answer 13

Answer: B
→ Treat as g:
C: 40.00 g / 12.01 (mw) = 3.331 mol
H: 6.72 g / 1.008 (mw) = 6.667 mol
O: 53.29 g / 16.00 (mw) = 3.331 mol
→ Divide by smallest (3.331):
C = 1, H = 2, O = 1 → CH₂O

Question 14

If the empirical formula is CH₂O and the molar mass is 180 g/mol, what is the molecular formula? (Molar mass of CH₂O = 30 g/mol)
A. CH₂O
B. C₆H₁₂O₆
C. C₂H₄O₂
D. C₄H₈O₄

Answer 14

Answer: B
→ given 180 / 30 (mw of compound)= 6 → (CH₂O)₆ = C₆H₁₂O₆

Question 15

Which of the following has the same empirical and molecular formula?
A. C₆H₁₂O₆
B. H₂O
C. C₂H₂
D. C₂H₄O₂

Answer 15

Answer: B
✔️ Water (H₂O) cannot be simplified.

Question 16

If a student produced 2.5 g of product but the theoretical yield was 3.0 g, what is the percent yield?
A. 16.6%
B. 83.3%
C. 120%
D. 60%

Answer 16

Answer: B
✔️ % Yield = (2.5 / 3.0) × 100 = 83.3%

Question 17

If a reaction was supposed to produce 3.5 g of H₂ but only 2.8 g was collected, what is the percent error?
A. 20%
B. 0.7%
C. 80%
D. 16.7%

Answer 17

Answer: A
✔️ % Error = |2.8 – 3.5| / 3.5 × 100 = 20%

Question 18

In the equation: Mg + 2HNO₃ → Mg(NO₃)₂ + H₂, how many grams of H₂ are produced from 2.00 mol Mg? (H₂ molar mass = 2.0 g/mol)
A. 2.0 g
B. 3.0 g
C. 4.0 g
D. 1.0 g

Answer 18

Answer: C
HOW TO CAL THEORETICAL MW AND H(2)
✔️ 2 mol Mg → 2 mol H₂ → 2 × 2.0 = 4.0 g H₂

Question 19

How many mL of 6.0 M HCl is needed to make 100 mL of 1.0 M HCl?
A. 100 mL
B. 16.7 mL
C. 60.0 mL
D. 20.0 mL

Answer 19

Answer: D
✔️ M₁V₁ = M₂V₂ → (6.0)V₁ = (1.0)(100) → V₁ = 16.7 mL

Question 20

How much water was added if you diluted 30 mL of 4.0 M solution to 100 mL?
A. 30 mL
B. 70 mL
C. 100 mL
D. 130 mL

Answer 20

Answer: B
✔️ Water added = V₂ – V₁ = 100 – 30 = 70 mL

Question 21

What volume of 0.5 M NaOH is needed to neutralize 25 mL of 1.0 M HCl?
A. 12.5 mL
B. 25.0 mL
C. 50.0 mL
D. 100.0 mL

Answer 21

Answer: B
✔️ HCl + NaOH → H₂O + NaCl
1:1 ratio → M₁V₁ = M₂V₂ → (1.0)(25) = (0.5)(V₂) → V₂ = 50 mL

(1.0)(25) = (0.5)(V₂) → V₂ = 50.0 mL
✅ Answer: C

Question 22

What’s the ratio used in titration between H₂SO₄ and NaOH?
A. 1:1
B. 1:2
C. 2:1
D. 3:2

Answer 22

Answer: B
✔️ H₂SO₄ has 2 H⁺, NaOH has 1 OH⁻ → ratio = 2:1 → use 2M₁V₁ = M₂V₂

depending on number of hydrogens in the front will determine if what number to put in the front of formuka

Question 23

According to the kinetic molecular theory, gas particles:
A. Occupy significant volume and exert attractive forces
B. Move in circular orbits
C. Have elastic collisions and negligible volume
D. All move at the same velocity

Answer 23

Answer: C
✔️ Gas particles are assumed to have elastic collisions (move in a straight line) and infinitely small volume.
✔️ constant arbitrary motion
✔️ molecules do not influence each other, no attractions
✔️ in a collision equal and opposite attraction forces
✔️ all gas particles have the same kinetic energy and is directly proportional to the temp

Question 24

Under which conditions do gases behave most ideally?
A. Low temperature and high pressure
B. Low temperature and low pressure
C. High temperature and low pressure
D. High temperature and high pressure

Answer 24

Answer: C
✔️ Ideal gas = high T (fast-moving) & low P (less interaction)

Question 25

What are Root Mean Square react on ?

Answer 25

Molecular weight and temperature speed of gas is directly proportional to temp and inversely proportional to molar mass of a gas.

Question 26

How many moles of N₂ are in 5.60 L at STP? A. 0.25 mol B. 0.50 mol C. 1.00 mol D. 2.00 mol

Answer 26

Answer: B ✔️ 5.60 / 22.4 = 0.25 mol

Question 27

Which gas effuses fastest? A. O₂ B. Cl₂ C. Ne D. CO₂

Answer 27

Answer: C ✔️ Lower molar mass = faster effusion → Ne has the smallest molar mass.

Question 28

At constant T and P, as molecular weight increases, gas density: A. Increases B. Decreases C. Stays the same D. Double

Answer 28

Answer: A it is directly proportional based on the equation P x Mw = D x R x T ✔️ Higher MW = higher density

Question 29

According to Henry’s Law, gas solubility increases with: A. Decreased pressure B. Increased temperature C. Increased pressure and decreased temperature D. Decreased pressure and temperature

Answer 29

Answer: C ✔️ Gases that are most soluble at high pressure & low temperature

Question 30

Which relationship is described by Henry’s Law? A. Solubility ∝ Volume B. Solubility ∝ Pressure C. Solubility ∝ 1/Temperature D. Solubility ∝ Density

Answer 30

Answer: B ✔️ Solubility increases directly with pressure: S₁/P₁ = S₂/P₂

Question 31

Compare the rate of diffusion of H₂ (MW = 2) to O₂ (MW = 32). A. 4× faster B. 8× faster C. √16 = 4× faster D. √32 = 5.66× faster

Answer 31

Answer: C ✔️ Rate ∝ √(heavier / lighter) = √(32 / 2) = √16 = 4

Question 32

How many moles of gas are in a 5.0 L container at 2.0 atm and 300 K? A. 0.41 mol B. 0.49 mol C. 0.29 mol D. 1.00 mol

Answer 32

R constant is 0.0821** PV = nRT → n = PV / RT = (2.0)(5.0) / (0.0821)(300) = 10 / 24.63 ≈ 0.41 mol → ✅ A

Question 33

What volume would 3.00 mol of gas occupy at 1.5 atm and 273 K? A. 40.0 L B. 30.0 L C. 44.8 L D. 22.4 L

Answer 33

V = nRT / P = (3)(0.0821)(273) / 1.5 ≈ 67.15 / 1.5 ≈ 44.8 L → ✅

Question 34

A gas exerts 3.0 atm in a 2.0 L container at 400 K. How many grams of O₂ (MW = 32 g/mol) are present? A. 3.1 g B. 4.9 g C. 6.4 g D. 7.9 g

Answer 34

Answer: D n = PV / RT = (3)(2) / (0.0821)(400) = 6 / 32.84 ≈ 0.183 mol grams = mol × MW = 0.183 × 32 ≈ 5.86 g → Closest: D

Question 35

What is the density of CO₂ at 2 atm and 273 K? A. 3.56 g/L B. 2.93 g/L C. 5.45 g/L D. 4.80 g/L

Answer 35

formula** P x Mw = D x R x T = (2)(44) / (0.0821)(273) = 88 / 22.42 ≈ 3.93 g/L → not listed, so double check math: Actual: 88 / 22.42 = 3.92 g/L → closest = A

Question 36

A container holds a gas mixture made of: * 3 moles of N₂ * 1 mole of O₂ * 2 moles of Ar If the total pressure of the mixture is 5 atm, what is the partial pressure of O₂? A. 1.0 atm B. 0.5 atm C. 2.5 atm D. 0.83 atm

Answer 36

✅ Solution: 1. Total moles = 3 + 1 + 2 = 6 moles 2. Mole fraction of O₂ = 1 / 6 3. Pp = Ptotal× (mol O₂ / total mol) = 5 atm × (1/6) ≈ 0.83 atm