CALCULATIONS WITH CHEMICAL FORMULAS & EQUATIONS

CALCULATIONS WITH CHEMICAL FORMULAS & EQUATIONSChapter 31

Learning objectives To calculate the percentage composition of the elements

To determine the empirical formula

To determine the molecular formula

To determine the limiting reactant

To determine the percentage yield of chemical reactions To interpret a chemical equation at the molecular, molar and mass level.

To learn Avogadro number 2

Learning outcomes students will be able to

Solve quantitative problems involving moles, mass, number of molecules from a given chemical equation and percentage yield

Solve the qualitative problems related to define empirical formula, molecular formula and limiting reactant 3

3.1 Molecular mass & formula massMolecular MassThe sum of the atomic masses of all the atoms in a molecule of the substance.

Formula MassThe sum of the atomic masses of all atoms in a formula unit of the compound, whether molecular or not.

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Molecular mass & formula mass-Exercise [1]Find the formula masses of the following substances

a. methanol, CH3OHb. nitrogen trioxide, NO3c. potassium carbonate, K2CO3d. nickel phosphate, Ni3(PO4)25

3.2 The mole conceptMole, molThe quantity of a given amount of substance that contains as many molecules or formula units as the number of atoms in exactly 12 g of carbon-12.

Avogadros Number, NAAvogadro is a Italian chemist The number of atoms in exactly 12 g of carbon-12NA = 6.022 10236

Molar MassThe mass of one mole of substance.

For example:Carbon-12 has a molar mass of 12 g or 12 g/mol

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molar mass- exercise[1] What is the molar mass of H3PO4?

[2] What is the molar mass of NH4NO3?8

[3]Calculate the mass in grams of the following.

a. 0.15 mol Na b. 0.594 mol Sc. 2.78 mol CH2Cl2 d. 38 mol (NH4)2S9 molar mass- exercise

3.3 Mass percentages from the formula Percentage Composition: The mass percentage of each element in the compound.

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Mass percentages from the formula-exercise [1] A 1.836-g sample of coal contains 1.584 g C. Calculate the mass percentage of C in the coal.

[2] A 6.01-g aqueous solution of isopropyl alcohol contains 4.01 g of isopropyl alcohol. What is the mass percentage of isopropyl alcohol in the solution?11

3.4 Elemental analysisPercentage Composition: The mass percentage of each element in the compound.Elemental analysis: sample is analyzed for its elemental (C,H,N)

Instrument: Mass spectrometric, atomic spectroscopy 12

3.5 Determining formulas Determining the Empirical Formula

Beginning with percent composition:Assume exactly 100 g so percentages convert directly to grams.Convert grams to moles for each element.Manipulate the resulting mole ratios to obtain whole numbers.

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Empirical formula- Exercise [1] Hydroquinone is 65.4% C, 5.5% H, and 29.1% O, by mass. What is the empirical formula of hydroquinone?

[2] Acrylic acid has the composition 50.0% C, 5.6% H, and 44.4% O. What is its empirical formula?

[3] The composition of the malonic acid is 34.6% C, 3.9% H, and 61.5% O. What is malonic acids empirical formula?14

To determine the molecular formula:Compute the empirical formula weight.Find the ratio of the molecular weight to the empirical formula weight.

Multiply each subscript of the empirical formula by n.

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Exercise Benzene has the empirical formula CH. Its molecular weight is 78.1 amu. What is its molecular formula?

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3.6 Molar interpretation of a chemical equation You may interpret a chemical equation either in terms of numbers of molecules or in terms of number of moles

N2 + 3H2 2NH31 molecule N2 3 molecules H22 molecules NH3Molecular interpretation 1 mol N2 3 mol H22 mol NH3Molar interpretation 1X28g N2 3X 2.02g H22 X 17g NH3Mass interpretation

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3.7 Amounts of substances in a chemical reactionStoichiometryThe calculation of the quantities of reactants and products involved in a chemical reaction.

The coefficients in the equation, can be used to calculate the moles of products obtained from any given moles of reactant

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1.Convert grams of A to moles of AUsing the molar mass of A

2.Convert moles of A to moles of BUsing the coefficients of the balanced chemical equation

3.Convert moles of B to grams of B Using the molar mass of B

To find the amount of B (one reactant or product) given the amount of A (another reactant or product):

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Stoichiometry-Exercise [1] 2C4H10(g) + 13O2(g) 8CO2(g) +10H2O(g)What is the amount (in moles) of carbon dioxide produced from0.30 mol C4H10?

[2] C2H5OH(l) + 3O2(g) 2CO2(g) + 3H2O(l)What is the amount (in moles) of water produced from 0.69 mol C2H5OH?

[3] 4Fe(s) + 3O2(g) 2Fe2O3(s)How many moles of O2 are needed to produce 3.91 mol Fe2O3?20

3.8 Limiting reactant; theoretical & percentage yieldsLimiting ReactantThe reactant that is entirely consumed when a reaction goes to completion.

Once one reactant has been completely consumed, the reaction stops.

How can we determine the limiting reactant?

Use each given amount to calculate the amount of product produced. The limiting reactant will produce the lesser amount of product.

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Limiting reactant - Exercise [1] ZrCl4(g) + 2Mg(s) 2MgCl2(s) + Zr(s)How many moles of zirconium metal can be produced from a reaction mixture containing 0.20 mol ZrCl4 and 0.50 mol Mg?

[2] 4KO2(s) + 2H2O(l ) 4KOH(s) + 3O2(g)If a reaction vessel contains 0.25 mol KO2 and 0.15 mol H2O,what is the limiting reactant? How many moles of oxygen can beproduced?

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Checking outcomes To calculate the percentage composition of the elements

To determine the empirical formula

To determine the molecular formula

To determine the limiting reactant

To determine the percentage yield of chemical reactions To interpret a chemical equation at the molecular, molar and mass level.

To learn Avogadro number

23Thank you

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