|How do you find the mole ratio in a stoichiometry problem?|
|Question Date: 2019-11-21|
A mole ratio is a conversion factor between the number of moles of one substance to the number of moles of another, such as (1 mol of O2) / (3 mol of H2).
Typically, the amount of one substance is known but determining the amount of a second substance is desired. The mole ratio enables one to find the amount of the second.
The mole ratio is simply the ratio of the coefficients from the balanced chemical reaction. For example, in the reaction of
N2 + 3 H2 -> 2 NH3,
the conversion factor for NH3 produced from reaction of a given amount of H2 gas (assuming excess N2) is given by the ratio 2 / 3 (the coefficient in front of NH3 in the balanced reaction / the coefficient in from the H2).
Thus, the amount of NH3 produced for reaction of 2.5 moles of H2 with extra N2 is given by:
# NH3 = 2.5 H2 * (2 NH3 / 3 H2) = 1.67 moles.
In order to find the mole ratio in a stoichiometry problem, you need to divide the prefactors in the chemical reaction formula by one another. For example, if the chemical reaction is
2A + 3B --> A2B3, the mole ratio of A2B3 to 2A will be 1/2 (i.e. the prefactors as a ratio).
When we write chemical equations, we write them as moles.
You know that H2 + O2 makes H2O -
hydrogen and oxygen make water.
You need 1 mole of H2 and 1/2 mole of O2 to make 1 mole of H2O.
This is the same as saying you need 2 moles of H2 and 1 mole of O2 - a mole ratio of 2:1. That would make 2 moles of water.
Here's a link about this:
"In stoichiometry, we shift our unit from molecule to mole. According to this equation, we need two moles of hydrogen to react with one mole of oxygen. This is called the mole ratio. It is defined as the ratio of moles of one substance to the moles of another substance in a balanced equation.Jan 29, 2015".
The mole ratio is normally given as the number in front of the chemicals of a formula. So, for example, the equation:
6O2 + C6H12O6 -> 6CO2 + 6H2O
It means that six moles of oxygen gas plus one mole of glucose will combine to produce six moles of carbon dioxide and six moles of water.
The mole ratio in a stoichiometry problem is indicated by the subscripts in the chemical names given.
For example, water is H2O, where the 2 is a subscript, indicating that there are twice as many hydrogen atoms as there are oxygen atoms (subscript 1, not usually written). Thus one mole of water has 2 moles of hydrogen atoms and one mole of oxygen atoms.
The mole ratio of hydrogen to oxygen is 2:1.
Calculate the ratio of the stoichiometric coefficients of the chemicals you want the ratio of. For example, if I have a reaction
aA -> bB,
then the ratio of A to B is a/b.
In a stoichiometrically balanced chemical reaction, the numerical coefficient of each chemical represents their relative number of moles. So to find the mole ratio, you simply divide the relevant stoichiometric coefficients. For example:
2 H2+ O2--> 2 H2 O
This essentially says for every 2 moles of H2 and 1 mole of O2, you can produce 2 moles of H2O.
So if you desired the mole ratio of products/reactants, this would be 2/(2+1)= 2/3. If you wanted the mole ratio of H2 to O2, you would have 2/1.
The first step of finding the mole ratio in a stoichiometry problem is to write the chemical equation of the reaction. Make sure you include the written chemical formulas for both the reactants and products. After you write the equation, balance the equation by making sure that the number of each type of atom on the reactant side equals the number of each type of atom on the product side, so the total number of hydrogen atoms on the reactant side must equal to the total number of hydrogen atoms on the product side, same with the total number of oxygen atoms, and so on.
You balance the two sides by writing down numbers in front of the chemical formulas for each of the reactants and products (these numbers are called coefficients). Note that the coefficient can be one. Balancing these equations usually means you start with the atoms with more atomic mass first, and leave hydrogen to last.
An example of an unbalanced chemical equation is
H2O --> H2 + O2
After you balance the equation, you can take the coefficients in front of the chemical formulas and make ratios. The balanced equation for the aforementioned reaction would be
2H2O --> 2H2 + O2.
In this case, the mole ratio between H2O (water) and O2 (oxygen) would be 2:1, or 2 to 1, or 2/1, and the mole ratio between O2 (oxygen) and H2 (hydrogen) would be 1/2.
This process of writing a chemical equation, balancing the equation, and finding the mole ratio is the standard approach to stoichiometry problems.
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