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PV equals nRTThe Ideal Gas Law is used to relate the pressure, volume, temperature and amount of an "ideal" gas. Although many gases are not perfectly ideal in reality, you can usually use the Ideal Gas Law anyway. Here is how you solve these problems!

The Ideal Gas Law is: PV = nRT.

Where:

-- P is the pressure of the gas (in atmospheres, ATM)

-- V is the volume of the container (in liters, L)

-- n is the number of moles of gas in the container (in moles, mol)

-- R is Universal Gas Constant (which is 0.0820574587 L · ATM · K-1 · mol-1)

-- T is the temperature of the gas (in Kelvin)

In English that says that pressure times the volume equals the number of moles times the Gas Constant times the temperature. It also means that pressure and volume are inversely proportional. Also it means the temperature is directly proportional to both the pressure and the volume. The amount of substance is directly proportional to the volume and the pressure.

-SO HOW DO YOU USE IT?

First, you need to figure out what you know from the question, and what you need to find. Since "R" is just a known constant, the Ideal Gas Law has only 4 variables in it: P, V, n, and T. In order to use this equation to solve for something, you must know at least 3 of these! Figure out which ones you know. Be careful, sometimes the units will not be the same as what I've written above. For instance, if you have the number of grams of a substance, that can be used to find the number of moles (if you know the molar mass), which give you the variable "n." Once you know what you are solving for, isolate that variable by rearranging the equation. Here are some examples:

To solve for the number of moles, we have: n =PV/RT

or to solve for the temperature, we have: T =PV/nR

or to solve for the pressure, we have: P =nRT/V

or finally, to solve for the volume, we have: V =nRT/P

Now, we MUST make sure everything is in the correct units before we solve!

PRESSURE (P): The unit of pressure must be in units of ATM. The units of pressure can be given in many different units. However, to use the Ideal Gas Law, the best unit to use is called an atmosphere, written "ATM." Here is how to convert from other units of pressure to ATM:

1 ATM = 14.6959488 pounds per square inch (psi)

1 ATM = 29.9246899 inches of Mercury (in Hg)

1 ATM = 760 mm mercury (mm Hg)

1 ATM = 760 torr (torr)

1 ATM = 101,325 pascals (Pa)

1 ATM = 101.325 kilopascals (kPa)

1 ATM = 1.01325 bar (bar)

If you see any of these other units of pressure being used, convert them to ATM using the factor given above.

--For example, if the problem give the pressure as 30.2 in Hg, do this:

30.2 in Hg ÷ 29.9247 in Hg/ATM = 1.009 ATM

-VOLUME (V): The volume must be in units of liters (L). Here are is the conversion from some other standard units of volume:

1 L = 1000 milliliters (mL)

1 L = 1000 cubic centimeters (cm3)

1 L = 1 cubic decimeter (dm3)

1 L = 0.001 cubic meters (m3)

1 L = 0.264172051 US gallons (G)

1 L = 1.0566882 US quarts (Q)

--For example, if the volume is 1 gallon, do this:

1 gallon ÷ 0.26417 G/L = 3.785 L

-NUMBER OF MOLES (n): The number of moles must be in units of moles. If not given in terms of moles, the most common way is to give it in number of grams. To go from grams to moles, divide the number of grams by the molar mass (MM) of the substance. Use the Periodic Table to calculate the molar mass.

number of grams / molar mass = number of moles

--For example, if there are 10 g of water, do this:

The molecular formula for water is H2O. The molar mass is then two times the molar mass of H plus the molar mass of O:

2*1.008 + 15.999 = 18.015 grams per mole

If there are 10 grams, to convert that to moles:

10 g ÷ 18.015 g/mol = .5551 moles

-UNIVERSAL GAS CONSTANT (R): You want to use this value: 0.0820574587 L · ATM · K-1 · mol-1. There are several different values of the Universal Gas Constant, but the only difference is what units they are in. Make sure you use the correct value. If you use another value, the other units will not match up correctly.

-TEMPERATURE (T): The units of temperature must be in Kelvin. You cannot use Centigrade (Celsius) or Fahrenheit. To convert:

-From degrees Celsius: add 273.15 to get Kelvin

-From degrees Fahrenheit: convert to Celsius first, then follow instructions above. To convert °F to °C, use this method:

-first subtract 32 from the °F number

-then divide that number by 9

-then multiply that by 5

Don't forget to convert °C to Kelvin afterwards!

--For example, if the temperature is 75 °C, in Kelvin that is:

75 °C + 273.15 = 348.15 K

Or instead, if the temperature is given as 150 °F, do this:

150 - 32 = 118

118 ÷ 9 = 13.111

13.111 * 5 = 65.556 °C

Then to get Kelvin: 65.556 °C + 273.15 = 338.706

-Once you've made sure all the units are correct, just plug in to the equation where you isolated the variable you want to find, and solve!

-- FINAL NOTE: Some questions say that the conditions are "at STP." That means that they are at Standard Temperature and Pressure.

-Standard temperature is equal to 0 °C, which is 273.15 K.

-Standard pressure is equal to 1 ATM.

ALSO: See the Related Questions links to the left of this answer. There are some example problems that have been solved correctly and show the details of the work. After reading this, look at those examples.

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