Stoichiometry homework

Stoichiometry: the study in which amount of reactant needed in each reaction to form product or the amount of product formed in each reaction. general quantities which are measured in this branch are Pressure(P), Volume(V), Temperature at which the reaction would occur and the number of moles(n) involved in the reaction. This branch is necessary as it tells that how much moles would react with other reactant to form desired product. As stated previously that a small change is reactants’ concentration could result a major change in type of product. Hence the stoichiometry deals with the number of reactants.

GENERAL EQUATION INVOLVED: PV = nRT where R→ Universal Gas Constant

GENERAL TERMS INVOLVED: Stoichiometric coefficient

STOICHIOMETRIC COEFFICIENT: it is the number written before a reactant species in a chemical reaction. This number represents the number of entities(moles) reacting with other reactants in a reaction. Illustration with an example: CH4 + 2O2 → CO2 + 2H2O. the Number ‘2’ ahead of O2 on the reactant side shows that 2 moles of O2 are reacting with 1 mole of CH4 to form 1 mole of CO2 and 2 moles of H2O. once we have the concentration of the reactants we can easily find out the amount of product formed.

Following are some of the topics in Basic Chemistry in which we provide help:

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Solving Stoichiometry Problems

Objectives:

    1.    Name four major categories of stoichiometry problems.

    2.    Explain how to solve each type of stoichiometry problems.

Notes:

It is important to remember that solving stoichiometry problems is very similar to following a recipe.  Once you know the recipe you can modify it using the same ratios to make the product for more or less people.

There are 4 major categories of stiochiometry problems.  It is important to remember, though, that in every situation you need to start out with a balanced equation.

1.    Mole-Mole Problems

Problem:  How many moles of HCl are needed to react with 0.87 moles of Al?

        Step 1: Balance The Equation & Calculate the Ratios

            2Al:6HCl (1:3)        2Al:2AlCl₃ (1:1)        2Al:3H₂ (1:1.5)

        Step 2:  Find the Moles of the Given

            0.87 moles of aluminum are reacted with hydrochloric acid

          Step 3: Calculate the moles using the ratios

            moles HCl = 0.87molAl x  3molHCl/1molAl = 2.6 mol HCl

           

2.    Mass-Mass Problems (Strategy: Mass g Mole g Mole g Mass)

Problem:  How many grams of Al can be created decomposing 9.8g of  Al₂O₃?

        Step 1: Balance The Equation & Calculate the Ratios

 

            2Al₂O₃:4Al (1:2)        2Al₂O₃:3O₂ (1:1.5)

Step 2:  Find the Mass of the Given

            9.8g Al₂O₃ are decomposed

Step 3: Calculate the moles of the given (mol/g)

            9.8g Al₂O₃ x (1mol Al₂O₃/102g Al₂O₃) = 0.096 mol Al₂O₃

       

 

Step 4: Calculate the moles using the ratios

        0.096 mol Al₂O₃ x (2 mol Al/1 mol Al₂O₃) = 0.19 mol Al 

      

        Step 5: Calculate the mass using the new moles

        0.19 mol Al x (27g Al/1 mol Al) = 5.1g Al

       

3.    Mass-Volume Problems (Strategy: Mass g Mole g Mole g Volume)

        Problem:  How many liters of H2 are created from the reaction of 20.0g K?

        Step 1: Balance The Equation & Calculate the Ratios

 

            2K:2H₂O (1:1)    2K:2KOH (1:1)    2K:1H₂ (2:1)

Step 2:  Find the Mass of the Given

            20.0g K are used in the reaction

Step 3: Calculate the moles of the given (mol/g)

            20.0g K x (1 mol K / 39g K) = 0.513 mol K

           

Step 4: Calculate the moles using the ratios

            0.51 mol K x (1mol H₂ /2mol K) = 0.266mol H₂

           

        Step 5: Calculate the volume using the new moles

        0.266 mol H₂ x (22.4L H₂ /1mol H₂) = 5.75L H₂

       

4.    Volume-Volume Problems

        Problem:  How many liters of SO₂ will be produced from 26.9L O₂?

Step 1: Balance The Equation & Calculate the Ratios

            2O₂:1S₂ (2:1)    2O₂:2SO₂ (1:1)    

Step 2: Find the volume of the given

            26.9L O₂

Step 3: Calculate the moles of the given

            26.9L O₂ x (1 mol O₂ / 22.4L) = 1.20 mol O₂

       

Step 4: Calculate the moles using the ratios

            1.20 mol O₂ x (1mol SO₂ /1mol  O₂) = 1.20 mol SO₂

       

        Step 5: Calculate the volume using the new moles

        1.20 mol O₂ x (1mol SO₂ /1mol  O₂) x (22.4L /1mol) = 26.9L SO₂

       

Stoichiometry 

• Stoichiometry is derived from greek words, “Stoicheion'' means element and'' metron” means measure. Stoichiometry is the study of the quantitative aspects of a chemical change. 

• Stoichiometry is the calculation of the quantities of reactants and products involved in a chemical reaction. In stoichiometry we try to find the masses, volumes, number of particles, energies etc.. of reactants taking part in a chemical reaction and products formed. 

• In stoichiometry we always use a balanced chemical equation because an unbalanced chemical equation violates the Law of “conservation of mass”.

• Some basic relations are:  

            1 mole of a gas = GMW of a gas = 22.4 L at STP

            1 mole of a substance = GMW

• For Homo atomic molecules GMW = Atomicity x GAW

• For Hetero atomic molecules GMW = sum of Gram atomic weight of all constituents.It is based on the chemical equation and on the relationship between mass and moles.

• The Avogadro number can be used to calculate the number of atoms or molecules in the given moles of the substances. The expression can be write as:
   

Stoichiometry Questions 

 Question 1: Calculate the number of atoms in 12 g of oxygen gas.  

Solution: 

Molar mass of oxygen gas = 32.0 g/mol

Number of atoms in 1 oxygen gas molecule = 2
 

 Question  2: Stoichiometry is derived from greek words, “Stoicheion” means element and“ metron” means _________.

(a) Compound

(b) Calculation

(c) Value

(d) Measure

Solution : Stoichiometry is derived from greek words, “Stoicheion” means element and“ metron” means measure

 Answer : (d)

Question 3: 1 mole of any gas   at STP occupies ___________.

(a) 32 litre

(b) 44 litre

(c) 22.4 litre

(d) All the above

Solution : 1 mole of any gas at STP occupies 22.4 litres. 

 Answer : (c)

Frequently Asked Questions

How do you calculate stoichiometry?

There are some guidelines to calculate stoichiometry. 

Assign 1 as the coefficient for the most complex species.

Balance the single-element species in last.

Eliminate fractional coefficients.

While balancing add coefficients only, do not change the chemical formula of any compound.

The number of atoms should be the same on both sides of the reaction.

For example- Let’s balance this equation using guidelines

The equation is not balanced as the number of N and O atoms is more on left (reactants) side

Assigning 1 coefficient in front of the most complex species

Balancing single element compounds

What is stoichiometry in chemistry?

Stoichiometry is the quantitative relationship between the number of moles of different reactants and products in a chemical reaction. We know that a chemical equation must be balanced and must have the same number of different atoms in the reactants and products. To balance a chemical equation we add coefficients in front of reactants and products. These coefficients are known as stoichiometric coefficients and their study is called stoichiometry. 

 
Why is stoichiometry so hard?

Stoichiometry problems can have multiple steps, which is not easy to memorize. Basic mathematics skills are required for stoichiometric calculations. Not understanding concepts can make it even more difficult. However, practicing solving problems after learning the concept well makes stoichiometry a lot more fun and interesting.

How do you do stoichiometry for dummies?

There are four simple steps to calculate stoichiometry:

• Convert the units of given substance to moles.

• Use mole ratio and calculate the moles of substance yielded in the reaction.

• Convert the moles of wanted substance to desired units.

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