# Chemistry/Density Marking Scheme

**MARKING SCHEME FOR : CALCULATING THE DENSITY OF WATER, A BLOCK OF WOOD AND A ROCK SAMPLE**

**Maximum = 9**

**DATA:**

- mass of 25 mL graduated cylinder = __ 45.02 g ___

- mass of graduated cylinder + 25.0 mL of water = ___ 69.79 g ___

- (1) number of rock sample = ___ 6 ___

mass of rock sample = ___ 44.22 g ___

- volume of water in 250 mL graduated cylinder = ___ 70 mL ___

- volume of water in 250 mL graduated cylinder after adding rock = ___ 77 mL ___

- what happens to rock when placed in water: ___ it sinks ___

- number of wood block = ___ 6 ___

- mass of wood block = ___ 32.96 g ___

- dimensions of wood block: length = ___ 6.9 cm ___

- width = ___ 3.9 cm ___

- height = ___ 2.5 cm ___

- width = ___ 3.9 cm ___

- what happens to wood block when placed in water: ___ it floats ___

**DISCUSSION:**

* IMPORTANT NOTE:* In all the questions and calculations that follow, you

**must**say what you are doing or calculating. If numbers are being subtracted or multiplied together the numbers involved in the calculation

**must**be shown and the result written down; if a special equation is being used the equations

**must**be shown, the numbers being used

**must**be shown as they are used in the equation and the results of the calculation

**must**be shown.

**1. (a) Calculate the mass of the water in the 25 mL graduated cylinder.**

(1) mass of water = 69.79 – 45.02 = **24.77 g**

**(b) Calculate the density of the sample of distilled water using the following formula. Round your answer off to THREE decimal places.**

[math]density = \frac {mass of water} {volume of water}[/math]

(1) [math]density = \frac {mass of water} {volume of water}[/math]
= [math]\frac{24.77g} {25mL}[/math] = 0.991g/mL

**2. (a) Calculate the volume of the rock sample.**

(1) Volume of rock = 77 – 70 = **7 mL**

**(b) Calculate the density of the rock sample using the following formula. Round your answer off to TWO decimal places.**

[math]density = \frac {mass of rock} {volume of rock}[/math]

(1) [math]density = \frac {mass of rock} {volume of rock}[/math] = [math]\frac{44.22g} {7mL}[/math] = 6.3g/mL

**3. (a) Calculate the volume of the wood block. The units are “cm3 ”. What is this volume when expressed in “mL”?**

(1) volume = 6.9 cm x 3.9 cm x 2.5 cm = 67.275 cm3 = **67.275 mL**

**(b) Calculate the density of the wood block using the following formula. Round your answer off to THREE decimal places.**

[math]density = \frac {mass of wood} {volume of wood}[/math]

(1) [math]density = \frac {mass of wood} {volume of wood}[/math] = [math]\frac{32.96g} {67.275mL}[/math] = = **0.490 g/mL**

(1) **4. Fill in the following table.**

Object | Density of Object | Density of Water | Does object float or sink in water? | Is object’s density greater or less than water’s density? |
---|---|---|---|---|

rock | 6.3 | 0.991 | sink | greater |

wood | 0.490 | 0.991 | float | less |

5. Write out and complete the following sentences using the information in the above table.

(1) **Objects having a density greater than the density of water will __ SINK __ when placed in water. Objects having a density less than the density of water will ___¬FLOAT __ when placed in water.**