We’re being asked to determine the volume of the gas sample collected over water at **STP** (standard temperature and pressure).

**At STP: ****T = 0****°****C**

** ****P = 1 atm**

We’re going to calculate the volume of the gas using the **ideal gas equation**.

$\overline{){\mathbf{P}}{\mathbf{V}}{\mathbf{=}}{\mathbf{n}}{\mathbf{R}}{\mathbf{T}}}$

P = pressure, atm

V = volume, L

n = moles, mol

R = gas constant = 0.08206 (L·atm)/(mol·K)

T = temperature, K

**The amount of gas (n) will not change and R is a constant:**

$\mathit{P}\mathit{V}\mathbf{=}\overline{)\mathbf{n}\mathbf{R}}\mathit{T}\phantom{\rule{0ex}{0ex}}\frac{\mathbf{P}\mathbf{V}}{\mathbf{T}}\mathbf{=}\frac{\overline{)\mathbf{T}}}{\overline{)\mathbf{T}}}\phantom{\rule{0ex}{0ex}}\frac{\mathbf{P}\mathbf{V}}{\mathbf{T}}\mathbf{=}\mathbf{1}$

**Relating the two conditions:**

A 500 mL gas sample is collected over water at a pressure of 740 mmHg and 25°C. What is the volume of the dry gas at STP? (STP= 1 atm and 0°C.)

Vapor Pressure of H_{2}O at 25°C: 24 mmHg

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Our tutors have indicated that to solve this problem you will need to apply the Collecting Gas Over Water concept. If you need more Collecting Gas Over Water practice, you can also practice Collecting Gas Over Water practice problems.

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