ch 3 mass relations in chemistry; stoichiometry. atomic mass indicates how heavy an element is...

CH 3 Mass Relations in Chemistry; Stoichiometry

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CH 3 Mass Relations in Chemistry;Stoichiometry

Atomic Mass

Indicates how heavy an element is compared to another element.

Units AMU---Atomic Mass UnitDefined as 1/12 of the mass of a C-12 atom

Atomic Mass from isotope composition

Isotopic Abundance: the natural of an isotope.

Isotope Atomic Mass Percent

abundance

Contribution to atomic mass

Ne-20 20.00 90.92 =

Ne-21 21.00 00.26 =

Ne-22 22.00 08.82 =

Page 4: CH 3 Mass Relations in Chemistry; Stoichiometry. Atomic Mass Indicates how heavy an element is compared to another element. Units AMU---Atomic Mass Unit

Mass of individual atoms

Mass of 1 atom = molar mass/ NA

(Avogadro's #)

Reacquaint yourself with the mole wheel.

Page 5: CH 3 Mass Relations in Chemistry; Stoichiometry. Atomic Mass Indicates how heavy an element is compared to another element. Units AMU---Atomic Mass Unit

The Mole

1 mol= 6.022 x 1023 items

1mol H = 6.022 x 1023 H atoms = 1.008g 1mol Cl= 6.022 x 1023 Cl atoms = 35.45g 1mol Cl2= 6.022 x 1023 Cl2 molecules = 70.90g

Page 6: CH 3 Mass Relations in Chemistry; Stoichiometry. Atomic Mass Indicates how heavy an element is compared to another element. Units AMU---Atomic Mass Unit

Molar mass (MM)

Molar mass is numerically equal to the sum of the atomic masses.

Page 7: CH 3 Mass Relations in Chemistry; Stoichiometry. Atomic Mass Indicates how heavy an element is compared to another element. Units AMU---Atomic Mass Unit

Mass % from formula

% composition of K2CrO4

Use part / whole, assume you have 1 mole of compound. (the math is easier)

1mol = K2CrO4 194.20 g

%K = 78.90/194.20 = %Cr =%O =

Page 8: CH 3 Mass Relations in Chemistry; Stoichiometry. Atomic Mass Indicates how heavy an element is compared to another element. Units AMU---Atomic Mass Unit

Simplest / empirical formula

Simplest whole number ratio of atoms present in a compound.

Page 9: CH 3 Mass Relations in Chemistry; Stoichiometry. Atomic Mass Indicates how heavy an element is compared to another element. Units AMU---Atomic Mass Unit

Simplest (empirical) formulafrom % composition

Steps:1. Find the mass of each element in the

sample compound, assume 100g total.

2. Find the numbers of moles of each compound.

3. Divide each by the smallest # of moles and look for obvious ratios.

Use the following K= 26.6%, Cr= 35.4%, O = 38.0%

Page 10: CH 3 Mass Relations in Chemistry; Stoichiometry. Atomic Mass Indicates how heavy an element is compared to another element. Units AMU---Atomic Mass Unit

Simplest formula (empirical) from analytical data

An organic sample containing only C, H, O atoms weighs 1.000g

Burning the sample gives 1.466g CO2,

0.6001g H2O

Find the simplest formula…

All the carbon from the sample is “locked up” in CO2

The portion of CO2 that is carbon can be determined by the mass ratio in the formula (12.01/44.01)

This multiply this by the mass of CO2 and you find grams of carbon in the sample.

Page 11: CH 3 Mass Relations in Chemistry; Stoichiometry. Atomic Mass Indicates how heavy an element is compared to another element. Units AMU---Atomic Mass Unit

Yield of product in a reaction

Ordinarily, reactants are not present in the exact ratio required for reaction.Usually 1 is in excess; some left when reaction

is over.

1 is limiting; completely consumed to give the theoretical yield of product.

Page 12: CH 3 Mass Relations in Chemistry; Stoichiometry. Atomic Mass Indicates how heavy an element is compared to another element. Units AMU---Atomic Mass Unit

Calculating theoretical yield

1. Calculate the yield expected if the first reactant is limiting.

2. Repeat the calculation for the second reactant

3. The theoretical yield is the SMALLER of these two quantities. The reactant that gave the smaller theoretical yield is the limiting reactant.

Page 13: CH 3 Mass Relations in Chemistry; Stoichiometry. Atomic Mass Indicates how heavy an element is compared to another element. Units AMU---Atomic Mass Unit

2 Ag (s) + I2 (s) 2 AgI (s)

Calculate the theoretical yield of AgI and determine the limiting reactant.

There is 1.00g Ag, and 1.00g I2.

Page 14: CH 3 Mass Relations in Chemistry; Stoichiometry. Atomic Mass Indicates how heavy an element is compared to another element. Units AMU---Atomic Mass Unit

% Yield

Suppose the actual yield is 1.50g AgI, what was the % yield?

Actual/Theoretical (x100) = % Yield

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