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Using Stoichiometry to find the composition of a mixture

September 24, 2019


A 13.20g sample of a mixture of calcium carbonate
(CaCO3) and sodium bicarbonate (NaHCO3) was heated and the compounds decomposed according
to these two reactions. So we have a mixture we started with that and ended up with this.
The decomposition of the sample yields 4.35g of CO2 and .873g of water (H2O). What percentage
by mass of the original sample was calcium carbonate (CaCO3)? Ok we are given this 2
amounts but this are amounts of products not reactants so this is not you might get confused
this is not a limiting reactant problem because we weren’t given amounts of reactant were
given amounts of two products. So here is the problem what are we starting with? We
want to figure out how much calcium carbonate (CaCO3) or what percent of this original mixture
was calcium carbonate (CaCO3). Well, if we start with 4.35g of
CO2 I don’t know how much of the CO2 came from the sodium bicarbonate (NaHCO3) and I
don’t know how much of the CO2 came from calcium carbonate (CaCO3) so I can not start with
CO2. You noticed that in this second equation water is produced and were given the amount
of water (H2O) but water is only produced in the second equation. So if I start with
.873g of water and I work back, I figure out how many grams of sodium bicarbonate (NaHCO3)
I had in the mixture I can subtract from 100 percent and get calcium carbonate. Ok So I
need to change this to moles. We know there’s 18.02 g of water per one mole of water. And
know I need my mole ratio and its 1 to 1. 1 mol H2O underneath 2 mol of sodium bicarbonate
(NaHCO3). Alright, and now I can figure out grams of sodium bicarbonate produced. I got
my handy molar mass calculator and its 84.01 grams per mole. So that’s going to give me
the mass of sodium bicarbonate (NaHCO3). What I’m going to do that’s has to be less than
13.20 when I get this answer. So what I’m going to do is multiply by 1 over 13.20g of
mixture. I could of just taken this answer and divided by 13.20 and then multiply by
100 to should give me the percent that is Sodium bicarbonate (NaHCO3). Alright, so .873
divided by 18.02, times 2, times 84.01 divided by 13.20 times 100. This equals to 61 and
I have how many digits 3 to work with 61.7 percent sodium bicarbonate (NaHCO3). So 100
– that is going to equal the percent that is calcium carbonate
(CaCO3) and that’s my answer.

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