Density: Quiz

Density: Quiz

7 questions

1. Question 1EasyWhich formula gives density?

   Aρ = m × VBρ = m / VCρ = V / mDρ = m − V
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   Answer: ρ = m / V

   Density is mass per unit volume, so ρ = m / V. Check the units: g ÷ cm³ gives g/cm³.

2. Question 2EasyA 54.0 g sample has a volume of 20.0 cm³. What is its density?

   A0.370 g/cm³B1.08 g/cm³C2.70 g/cm³D1080 g/cm³
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   Answer: 2.70 g/cm³

   ρ = 54.0 g ÷ 20.0 cm³ = 2.70 g/cm³. 0.370 is the fraction upside down (cm³/g).

3. Question 3EasyWater in a measuring cylinder rises from 30.0 mL to 45.2 mL when a metal piece is added. What is the volume of the metal?

   A15.2 mLB30.0 mLC45.2 mLD75.2 mL
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   Answer: 15.2 mL

   The volume of the object is the rise in level: 45.2 mL − 30.0 mL = 15.2 mL (= 15.2 cm³).

4. Question 4MediumWhich object will float in water (0.998 g/cm³)?

   AMass 20.0 g, volume 25.0 cm³BMass 30.0 g, volume 25.0 cm³CMass 50.0 g, volume 10.0 cm³DMass 12.0 g, volume 4.00 cm³
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   Answer: Mass 20.0 g, volume 25.0 cm³

   20.0 g ÷ 25.0 cm³ = 0.800 g/cm³, less than water, so it floats. The others are 1.20 g/cm³, 5.00 g/cm³ and 3.00 g/cm³: all sink.

5. Question 5MediumEthanol has a density of 0.789 g/mL. What is the mass of 200. mL of ethanol?

   A158 gB253 gC0.00395 gD200. g
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   Answer: 158 g

   m = ρV = 0.789 g/mL × 200. mL = 157.8 g, which is 158 g to 3 significant figures. 253 g comes from dividing (200. ÷ 0.789).

6. Question 6MediumWhy does ice float on liquid water?

   AIce is lighter than waterBIce is less dense than liquid waterCIce contains trapped airDCold objects always float
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   Answer: Ice is less dense than liquid water

   Ice (0.917 g/cm³) is less dense than liquid water (about 1.00 g/mL) because water molecules are held further apart in the ice structure. Floating depends on density, not on mass.

7. Question 7HardFive measurements of a metal density are 2.68, 2.71, 2.69, 2.72 and 2.70 g/cm³. Which is the best report?

   A2.70 ± 0.02 g/cm³ (n = 5)B2.72 g/cm³C2.70 ± 0.04 g/cm³ (n = 5)D13.50 g/cm³
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   Answer: 2.70 ± 0.02 g/cm³ (n = 5)

   Mean = 13.50 g/cm³ ÷ 5 = 2.70 g/cm³. Sample standard deviation s = 0.016 g/cm³, which rounds to 0.02 g/cm³. ±0.04 is the range (2.72 − 2.68), not the standard deviation.