# When using the formula PV = nRT, why is R = 82.05746? An example of when using this formula is below, which is a question I have previously asked already.What is the amount of hydrogen released in...

When using the formula PV = nRT, why is R = 82.05746? An example of when using this formula is below, which is a question I have previously asked already.

What is the amount of hydrogen released in the following case:

Silicon steel is an alloy of the elements iron, carbon and silicon. An alloy sample was reacted with excess hydrochloric acid and the following reaction occurred:

Fe (s) + 2HCl (aq) ---> FeCl2 (aq) + H2 (g)

The carbon and silicon in the alloy did not react with the acid. If an alloy sample with a mass of 0.160 g produced 62.0 mL of hydrogen gas, measured at SLC, calculate the amount of hydrogen evolved in the reaction.

When finding the answer by using the formula of PV = n**R**T, why is **R = 82.05746**? I thought it was just **8.31**, please explain to me why this is so..

The answer to this question was:

When a sample of silicon steel that is made up of iron, carbon and silicon is reacted with excess hydrochloric acid the following reaction occurs:

Fe (s) + 2HCl (aq) ---> FeCl2 (aq) + H2 (g)

The mass of the sample used is 0.16 g and the volume of hydrogen produced when measured at standard laboratory conditions with a temperature of 25 C and a pressure of 1 atm. is 62 mL

The amount of hydrogen released can be calculated using the ideal gas law, PV = n*RT, where P is the pressure, V is the volume, n is the amount of the gas in moles, T is the temperature and R is a constant.

1*62 = n*82.05746*298

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### 1 Answer

The volume (V) occupied by a specific number of moles (n) of a gas at a temperature (T) and a pressure (P) is related by the ideal gas law as P*V = n*R*T

R is the universal gas constant and has a value that is dependent on the units that the other variables are expressed in. The pressure can be expressed in terms of atm, bar, Pa, Torr, etc. Similarly, volume can be given in terms of mL, L, cm^3, m^3, etc. When the ideal gas law is being used to estimate any one of the variables when the others are known it is important to ensure that an appropriate value for R is used. For example R = 8.314 would be used to measure volume in m^3 if the pressure is given in pascal. On the other hand, if the volume has to be determined in cm^3 and the pressure is given in terms of atm, the value of R to be used is 82.05746.

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