# Help Zone

### Student Question

Hi! I have a lab this week on the density of gases, liquids, and solids.

I understand that for the liquids I can weigh and measure the volume, but I have two questions:

1) how do I measure the mass of a gas?

2) how do I measure the volume of a solid?

Science ## Explanations (1)

• Explanation from Alloprof  Explanation from Alloprof

This Explanation was submitted by a member of the Alloprof team.

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Team Alloprof • 9mo. edited September 2021

Hi!

Thank you for your question 🙂

First, to calculate the density of a gas in a laboratory, the volume of the gas must be measured using a syringe. But, be careful. To record the volume of a gas contained in a syringe, we must read the value associated with the location of the upper part of the plunger.

Then, to determine the mass of the gas contained in the syringe, you must follow a few steps.

You may start by weighing the syringe when there is no gas in it. To do this, you must empty the syringe, insert its stopper and then pull the plunger, up to the maximum volume. Then, the syringe can be filled with the unknown gas and be weighted again. To determine the mass of the gas, you need to subtract the mass of the syringe from the total mass of the syringe and the gas, as in the following formula:

$$m_{gas} = m_{seringue + gas} − m_{seringue}$$

Second, as we can't determine the volume of a solid with an irregular shape using a ruler, we must use a graduated cylinder and water. To do so, we fill up the cylinder with water, and we record that volume. Then, we carefully add the solid to the water contained in the graduated cylinder, and we note that value (water+solid).

Then, to determine the volume of the solid alone, we can subtract the volume of water from the total volume of water and solid, as in the following formula:

$$V_{solid} = V_{water + solid} − V_{water}$$