Question

The mass of a fully dried rock sample of volume 100 cm³ is 300 g. The mass of the sample, when fully saturated with water of density 1.00 g/cm³, is 325 g. Assuming no volume change, the computed porosity of the rock is ........ % (answer in one decimal place).

Hint:

To calculate porosity, subtract the dry mass from the saturated mass, divide by the density of water, and then divide by the total volume of the sample.

Answer 1

Correct Answer: 25

Step 1: Understand the porosity formula.
Porosity is the fraction of the rock's volume that is void space, and it can be calculated using the following formula: \[ \text{Porosity} = \frac{\text{Volume of voids}}{\text{Total volume}} \times 100 \] The volume of voids is the difference between the saturated mass and the dry mass, divided by the density of water.

Step 2: Calculate the volume of voids.
The mass of the rock when saturated is 325 g, and the mass when dry is 300 g. The volume of voids is: \[ \text{Volume of voids} = \frac{325 \, \text{g} - 300 \, \text{g}}{1.00 \, \text{g/cm}^3} = 25 \, \text{cm}^3 \]

Step 3: Calculate the porosity.
The total volume of the rock is 100 cm³, so the porosity is: \[ \text{Porosity} = \frac{25 \, \text{cm}^3}{100 \, \text{cm}^3} \times 100 = 25.0% \]

Step 4: Conclusion.
The computed porosity of the rock is 25.0%.