Chemistry/Density Marking Scheme

Science 8 Experiment
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 ___
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.

$density = \frac {mass of water} {volume of water}$

(1) $density = \frac {mass of water} {volume of water}$ = $\frac{24.77g} {25mL}$ = 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.

$density = \frac {mass of rock} {volume of rock}$

(1) $density = \frac {mass of rock} {volume of rock}$ = $\frac{44.22g} {7mL}$ = 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.

$density = \frac {mass of wood} {volume of wood}$

(1) $density = \frac {mass of wood} {volume of wood}$ = $\frac{32.96g} {67.275mL}$ = = 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.