# Calculating Moles Using the Ideal Gas Law

• kasse
In summary, using the ideal gas law, we can calculate the number of moles of gas when given the volume, pressure, and temperature. In this case, the correct answer is 0.114 moles. It is important to make sure that the units used for pressure match the units of the gas constant, which can vary depending on the units used. For example, the gas constant can be expressed as 10.7316 ft3·psi·°R-1·lb-mol-1 or 8.3145 J/(K*mol). Therefore, when using the ideal gas law, it is important to double check the units to ensure accurate calculations.

## Homework Statement

When the volume of a gas is 2.81 L, the pressure is 740 Torr and the temperature is 20oC, how many moles of gas is there?

PV=nRT

## The Attempt at a Solution

740 Torr = 740*133.3 = 98.6*10^3 Pa
Temperature in Kelvin: 293 K

n = PV/RT = (98.6*10^3 * 2.81) / (8.3145*293) = 114 moles

The corret answer is 0.114 moles. Does this mean that when using the ideal gas law, the pressure should be in kPa?

What are your units for R? The pressure unit must match that.

J/(K*mol)

Does this mean that when using the ideal gas law, the pressure should be in kPa?

All it means is that units of data must match units of R.

R can take any value, it all depends on the units used. For example - 10.7316 ft3·psi·°R-1·lb-mol-1. Don't ask me what lb-mol is

Check how J/(K*mol) relates to pressure in kPa. What is Pa?

kasse said:
J/(K*mol)

Hint: "Oh, I thought it was in L kPa/mol K!"

## 1. What is the ideal gas law and how is it used to calculate moles?

The ideal gas law is a mathematical equation that describes the relationship between pressure, volume, temperature, and the number of moles of a gas. It is written as PV = nRT, where P is pressure, V is volume, n is the number of moles, R is the ideal gas constant, and T is temperature. To calculate moles using this law, you would rearrange the equation to solve for n, which would give you the number of moles.

## 2. How do you determine the ideal gas constant, R?

The ideal gas constant, R, is a constant value that is determined experimentally and varies depending on the units used. The most commonly used value is 0.0821 L·atm/mol·K, but it can also be expressed in other units such as J/mol·K or m^3·Pa/mol·K. It is important to use the correct value of R depending on the units used in the ideal gas law equation.

## 3. Can the ideal gas law be used for all gases?

The ideal gas law is most accurate for gases at low pressures and high temperatures. At high pressures, the volume of the gas particles becomes significant and the ideal gas law becomes less accurate. Additionally, at low temperatures, the particles may start to condense and the gas may no longer behave ideally. In these cases, other equations such as the van der Waals equation may be used.

## 4. How do you convert between moles and other units of measurement?

To convert between moles and other units of measurement, you would first need to know the molar mass of the substance in question. This can be found by adding up the atomic masses of each element in the compound. Then, you can use dimensional analysis to convert between moles and other units such as grams or liters. For example, to convert from moles to grams, you would use the conversion factor 1 mol/ molar mass in grams.

## 5. What are the units for pressure, volume, temperature, and moles in the ideal gas law equation?

The units for pressure, volume, temperature, and moles in the ideal gas law equation depend on the units used for the ideal gas constant, R. If R is in units of L·atm/mol·K, then pressure must be in atmospheres, volume in liters, temperature in Kelvin, and moles in mol. If R is in other units, then the units for pressure, volume, temperature, and moles will also vary. It is important to use the correct units in the ideal gas law equation to get an accurate result.