9. stoichiometry

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What is stoichiometry ????

Stoichiometry is the quantitative study of reactants and products in a chemical reaction.

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Given : Amount of reactants Question: how much of products can be formed.

Example 2 A + 2B 3C

Given 20.0 grams of A and sufficient B, how many grams of C can be produced?

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What do you need???

You will need to use i. molar ratios, ii. molar masses, iii. balancing and interpreting equations, andiv. conversions between grams and moles.

Note: This type of problem is often called "mass-mass."

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Steps Involved in Solving Mass-Mass Stoichiometry Problems

Balance the chemical equation correctlyUsing the molar mass of the given substance, convert the mass given to moles. Construct a molar proportion (two molar ratios set equal to each other) Using the molar mass of the unknown substance, convert the moles just calculated to mass.

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Moles Ratio

A mole ratio converts moles of one compound in a balanced chemical equation into moles of another compound.

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example

Reaction between magnesium and oxygen to form magnesium oxide. ( fireworks)

2 Mg(s) + O2(g) → 2 MgO(s) Mole Ratios:

2 : 1 : 2

Reaction between magnesium and oxygen to form magnesium oxide. ( fireworks)

2 Mg(s) + O2(g) → 2 MgO(s) Mole Ratios:

2 : 1 : 2

1) N2 + 3 H2 ---> 2 NH3

Write the mole ratios for N2 to H2 and NH3 to H2.2) A can of butane lighter fluid contains 1.20 moles of butane (C4H10). Calculate the number of moles of carbon dioxide given off when this butane is burned.

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Using the practice question 2) above: Equation of reaction 2C4H10 + 13O2 8CO2 + 10H2O Mole ratio

C4H10 CO2 1 : 4 [ bases]

1.2 : X [ problem] By cross-multiplication, X = 4.8 mols of CO2 given off

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Problem 1:1.50 mol of KClO3 decomposes. How many grams of O2 will be produced? [k = 39, Cl = 35.5, O = 16]

2 KClO3 → 2 KCl + 3 O2

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Use mole ratioGet the answer in moles and thenConvert to Mass. [Simple Arithmetic]

Hello!If you are given a mass in the problem, you will need to convert this to moles first. Ok?

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2 KClO3 → 2 KCl + 3 O2

2 : 3 1.50 : XX = 2.25molConvert to mass2.25 mol x 32.0 g/mol = 72.0 grams

Cool!

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We want to produce 2.75 mol of KCl. How many grams of KClO3 would be required?Soln

KClO3 : KCl2 : 2X : 2.75X = 2.75mol

In mass: 2.75mol X 122.55 g/mol = 337 grams

zooo zimple!

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There are four steps involved in solving these problems:Make sure you are working with a properly balanced equation. Convert grams of the substance given in the problem to moles. Construct two ratios - one from the problem and one from the equation and set them equal. Solve for "x," which is usually found in the ratio from the problem. Convert moles of the substance just solved for into grams.

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Just follow mass-mass problem to the

penultimate level

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There are four steps involved in solving these problems:Make sure you are working with a properly balanced equation. Convert grams of the substance given in the problem to moles. Construct two ratios - one from the problem and one from the equation and set them equal. Solve for "x," which is usually found in the ratio from the problem. Convert moles of the substance just solved for into Volume.

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No of moles = VolumeMolar volume

Can you remember a similar equation?

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The molar volume is the volume occupied by one mole of ideal gas at STP. Its value is: 22.4dm3

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Calculate the volume of carbon dioxide formedat STP in ‘dm3' by the complete thermal decomposition of 3.125 g of pure calcium carbonate (Relative atomic mass of Ca=40,C=12, O=16)Solution: Convert the mass to mole:Molar mass of CaCO3

= 40 + 12 + (16 x 3) = 100gmol-1

Mole = mass/molar mass3.125/100 = 0.03125mol

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As per the equation,

Mole ratio 1 : 1problem 0.03125mol X

X = 0.03125mol of CO2

Convert mole to volume [slide 17]

Volume = (0.03125 x 22.4)dm3 = 0.7dm3

Molar Mass

• Molar mass (mm) states the mass of 1 mol of substance,

• Expressed in g/mol• The value of molar mass (mm) is the same as

the value of Ar or Mr• But with a unit g/mol

mm = Ar or Mr in g/mol

Molar mass (mm) is used to determine the number of moles in certain mass of substance.The relationship between the number of mole (n) and the mass of substance(w) is given by:

n = w mm

Example

Urea, whic is used afertilizer, has a chemical formula of CO(NH2)2

Show that its mm is equal to its Mr but expressed in g/mol.

Using the molar mass value, calculate the number of moles in 30 kg of urea.Ar C=12, O= 16, N=14, H=1

a. Known that....Mr urea: 60mm= 6, 02x1023 molecules x 60 sma x 1,66x10-24 g

mol molekul amu = 60 g/mol

The calculation shows that mm is the same as the value of Mr, expressed in different unit.

b. N= w = 30, 000 g = 500 mol mm 60 g/mol

• The percentage by mass of elements in a compound can be determined from its chemical formula as stated below:

• Percentage by mass of elemnts in the compound (%)=

(IN element x Ar element ) x 100%Mr compound

IN = The index number of element in the chemical formulaAr = the relative atomic mass of elementMr = the relative molecular or formula mass

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