# Stoichiometric calculation

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Stoichiometric calculationInformation obtained from Chemical EquationsA chemical equation tells us what type of chemical changes take place during a chemical reaction. While a balanced chemical equation also tells the molar relationships between one reactant to another, and between reactants and products. Consider the following balanced chemical equation:

CO(g) + 2H2(g)  CH3OH(l)Mole-to-Mole RelationshipsThe coefficients in balanced equation give mole relationships between reactants and products. Since mole quantity is directly related to mass, the equation also provide information regarding mass relationships between substances involved in the reaction. For example, the above equation implies that:

One CO molecule reacts with two H2 molecules to form one CH3OH molecule.Or,one mole of CO reacts with two moles of H2 to form one mole of CH3OH.

The combustion reaction of propane gas, C3H8(g), represented by the following equation,
C3H8(g) + 5 O2(g)  3 CO2(g) + 4 H2O(g)

implies that one mole of C3H8 reacts with 5 moles of O2 to yield 3 moles of CO2 and 4 moles of H2O.

The stoichiometric ratios in the above reaction are: 1 mol CH3OH and 1 mol CH3OH1 mol CO 2 mol H2
These mole ratios can serve as conversion factors in stoichiometric calculations.

Exercise-1:
1.For the combustion of propane gas according to the following reaction:
C3H8(g) + 5 O2(g)  3 CO2(g) + 4 H2O(g);
(a) How many moles of oxygen gas will react with one mole of propane?
(b) How many moles of CO2 and H2O, respectively, are formed from one mole ofpropane reacted?
(c) How many moles of O2 gas are required to react exactly with 0.25 mol of C3H8?
(d) How many moles of C3H8 are combusted if 3.75 moles of O2 were consumed during the reaction?.
(e) How many moles of CO2 and H2O, respectively, will be produced from the complete combustion of 0.25 mole of propane gas?

2.Ammonia is produced by the following reaction: N2(g) + 3 H2(g) 2 NH3(g)
(a) How many moles of ammonia are produced if 12.0 moles of H2 are completely reacted?
(b) How many moles nitrogen gas are needed to react completely with 12.0 moles of hydrogen gas according to the above reaction?

(c) How many moles each of N2 and H2, respectively, are needed to produce 12.0 moles of ammonia according to the above equation?____________________________________________________________

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Calculations of Masses
Mass-to-Mole-to-Mole-to-Mass RelationshipsIn any given reaction, if the mass of one of the reactants is given, its quantity in moles can be obtained. Then the moles of the other reactant or the product can be determined from the balanced equation. This quantity in moles of the other reactant or product(s) can be converted backto their respective masses.

Consider the formation of CH3OH from CO and H2 according to the equation:
CO(g) + 2 H2(g)  CH3OH(l)

Suppose that the reaction uses 454 g of CO gas, how many grams of hydrogen gas are also reacted, and how many grams of methanol, CH3OH, are formed?

Solution: First we have to convert 454 g of CO to moles of CO gas:
? Moles of CO = 454 g x 1 mol CO__ =16.2 mol CO;
28.01 g CO

Using the balanced equation, we find how many moles and grams of H2 that will react with 454 g of CO:
? Moles of H2 = 16.2 mol CO x 2 mol H2_ = 32.4 mol H2;
1 mol CO
? grams of H2 = 32.4 mol H2 x 2.016 g H2 = 65.3 g H2
1 mol H2

Next, we calculate the moles and grams of methanol that would be formed if the reaction occurs to completion. That is, all of COare reacted.

? Moles of CH3OH = 16.2 mol CO x 1 mol CH3OH = 16.2 mol CH3OH
1 mol CO
? Grams of CH3OH = 16.2 mol CH3OH x 32.04 g CH3OH = 519 g CH3OH
1 mol CH3OH
Exercise-2:
1.In the following reaction to produce ammonia: N2(g) + 3H2(g)  2NH3(g)
(a) How many grams of H2 gas are needed to react completely with 454 g of nitrogen gas? (b) How many grams of ammonia will be...