In an experiment involving the reaction shown below, 150 cm^3 of F_2 reacts with 100 cm^3 of Cl_2.
3F_2(g) + Cl _2(g) → 2Cl F_3(g)
On completion of the reaction, what is the final gaseous volume at the same temperature and pressure?
A. 250 cm^3
B. 100 cm^3
C. 350 cm^3
D. 200 cm^3
E. 150 cm^3
This is a question that many students struggle with, and it is actually simpler than you think.
Recall:
Avogadro’s Law
Equal volumes of all gases, at the same constant temperature and pressure, have an equal number of particles.
V∝n
At 1 atm and 0°C, 1 mole of gas has a volume of 22.4L
This means that we can base the stoichiometry of this question based off coefficients because there is a constant relationship between volume and moles for all of them.
Now onto the question, where we are told that 150 cm^3 of F_2 reacts with 100 cm^3 of Cl_2
3F_2(g) + Cl _2(g) → 2Cl F_3(g)
(150cm^3) (100cm^3)
If we look at their stoichiometric coefficients, they should be reacting in a 3:1 ratio.
This means that for every 3cm^3 of F_2(g) for every 1g of Cl _2(g). We also know that for every 3cm^3 of 3F_2(g) and 1g of Cl _2(g), we have 2cm^3 of Cl F_3 formed as a product.
So, 150cm^3 of F_2(g) will react with 50cm^3 of Cl_2(g), leaving 50cm^3 unreacted and in excess.
For products, we will have 100cm^3, since its 2cm^3 per 3cm^3 of 3F_2(g) and 1g of Cl _2(g).
Now, for our final answer, what is left at the end of the reaction?
Of course, we will have 100cm^3 of Cl F_3, however, we will also have all of the unreacted gas, which is 50cm^3 of Cl_2.
100cm^3 Cl F_3 + 50cm^3 Cl_2 = 150cm^3 total
Hi, I did not fully understand this part. How do we get the quantity of products?
Hi John, we need to use Avogadro’s law to get the products. Since this is a problem that has only gases reacting, we can use the knowledge that equal volumes of all gases have same number of particles (when temp and pressure constant). This means that we can take the volumes we are given in the question and compare them to the stoichiometric ratios. We have 3 F2(g) and 1 Cl2, for every 1cm3 of Cl2, we will have 3cm3 of F2 because the gases have the same particles at the same volume. Does this make more sense?