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Non Life Sciences => Chemistry => Topic started by: Indranil on 24/04/2018 07:58:38

Title: Why Mass <> moles<> no of molecules are interconverted?
Post by: Indranil on 24/04/2018 07:58:38

Here is a reaction below
CH4 (g) + 2O2 (g) = CO2 (g) + 2H2O (g)
in the rection above, the coefficients 2 for O2 and H2O and similarly the coefficient for CH4 and CO2 is one in each case.
Thus according to the above chemical reaction, I can write
1. one mole of CH4 (g) reacts with two moles of O2 (g) to give one mole of CO2 (g) and two moles of H2O (g)
2. one molecule of CH4 (g) reacts with two molecules of O2 (g) to give one molecule of CO2 (g) and two molecules of H2O (g)
3. 22.4 L of CH4 (g) reacts with 44.8 L of O2 (g) to give 22.4 L of CO2 (g) and 44.8 L of H2O (g)
4. 16 g of CH4 (g) reacts with 2 X 32 g of O2 (g) to give 44 g of CO2 (g) and 18 X 2 g of H2O (g)
Now my question is If 'mole' and 'molecule' are different things, then why these relationships can be interconverted? Could you explain it?

Title: Re: Why Mass <> moles<> no of molecules are interconverted?
Post by: evan_au on 24/04/2018 11:31:08

Equation #2 describes the following reaction (molecule by molecule):
CH₄ (g) + 2O₂ (g) = CO₂ (g) + 2H₂O (g)

Equations #1 (mole by mole), #3 (liter by liter) & #4 (gram by gram) are just ways of expressing the following reaction (where N_A is Avogadro's constant):
N_ACH₄ (g) + 2N_AO₂ (g) = N_ACO₂ (g) + 2N_AH₂O (g)
#1: Avogadro's number of molecules = 1 mole (regardless of whether is is solid, liquid or gas)
#3: Avogadro's number of molecules of an ideal gas takes up 22.4 liters at standard temperature & pressure
#4: Avogadro's number of molecules has a mass in grams which is equal to the atomic mass in grams (regardless of whether is is solid, liquid or gas)

These are some interlinked definitions.
#4 is the definition, which determines the value of N_A
#3 is a measurement, which can be extrapolated to other temperatures and pressures (provided the pressure doesn't get too high, or the temperature get low enough for the gas to start condensing into a liquid).

These are all ways of expressing Proust's Law, which dates from 1779:
https://en.wikipedia.org/wiki/Law_of_definite_proportions