What mass of KNO3?

Question

What mass of KNO3 would have to be decomposed to produe 21.1L of Oxygen measured at STP?

Answer 1

First the stoichiometry of the decomposition.

2 KNO3 --> 2 KNO2 + O2

Assume an Ideal gas, then rearange the ideal gas law (PV=nRT) to:

n = (PV) / (RT)

n = moles of gas (in mol)
P = Pressure (in kPa)
V = Volume (in L)
R = Universal Gas constant (in (L * kPa) /( K * mol))
T = Temperature (in K)

Now here you need to do some work, when I was in school STP (standard Temperature and Pressure) was defined as 273.15K and 101.325kPa. The current IUPAC definition says 273.15K and 100kPa. Check which STP to use.

Once you have settled on which pressure to use plug in the values into the above equation along with the volume given (21.1L). R = 8.314472 in units of (L kPa) / (K mol).

That gives you the number of moles of O2 produced.

Now look back at the stoichiometric equation and notice that 2 molecules of KNO3 react to form 1 molecule of O2. So multiply the number of moles of Oxygen by 2 to get the number of moles of KNO3.

Now that you know the number of moles of KNO3 multiply that number by it atomic weight (101.1 g/mol) to get the number of grams (the mass).

Answer 2

In all problems that involve balancing different substances, you are going to need moles and you are going to need a balanced equation.

2 KNO3 ---> 2 KNO2 + O2

The next thing is to look at the substance that you know most about, and work from that towards what you need for your answer.

So go for moles O2 using PV = nRT

Notice from the equation that each mole O2 corresponds to 2 moles KNO3

Now you know the number of moles KNO3, multiply by formula mass to get mass of KNO3, present for grading, collect full marks.

Answer 3

! mole of an iseal gas at STP occupies 22.4 L
21.1 L = 0.942 moles O2 (21.1 / 22.4)
KNO3 contains 3 O atoms per molecule. So 1 mole of KNO3 will yield 1.5 moles of O2.
0.942 / 1.5 = 0.629 moles of KNO3
0.629 moles x 101.11 g/mol = 63.6 g KNO3

0.629 mole KNO3 contains 3x0.629 moles of O atoms (1.887 moles of O atoms or 0.94 moles of O2 molecules)