# Chemistry Calculations by changcheng2

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```									Chemistry Calculations

This is where you apply the
Math that you have been
learning.
Density
 The amount of matter present in a
given volume of a substance.
 Formula: Density = mass/volume or
D=m/v
 Unit: g/mL or g/cm3
 1 mL = 1 cm3
Density Problems
   A student finds that 23.5 mL of a liquid
weighs 35.062 g. What is the density of
this liquid?
   A solid substance has a weight of 65.89 g
and a volume of 31.4 mL, what is the
density?
Density Problems on WB
1. If a substance has a volume of 24.67 mL
and a mass of 98.6 g, what is the density?
2. Using the following pieces of information,
determine the density: 14.8 mL and 84.78
g.
3. If a solid object has a mass of 75.9 g and a
volume of 23.9 cm3, what is the density?
How to Find Volume
 When the volume of a sample is unknown, it
can be determined by placing the solid in a
graduated cylinder and seeing how much
the water is displaced by the object.
 Final volume – initial volume = volume of
object
Using the Density Chart
 A student finds a medallion at a pawn shop.
The store owner tells the student that the
medallion is platinum, but the student
believes that it is silver. If the medallion
weighs 55.64g, has an initial volume = 75.2
mL and a final volume = 77.8 mL, what is
the density of the medallion and what is it
Using the Density Chart
 A piece of wood was found to have a mass
of 7.182 g, an initial volume of 24.6 mL and
a final volume of 37.9 mL. What is the
density of the wood and what is it made
from?
Using the Density Chart on WB
1. Mr. Jacobs has found a substance and needs to
know what it is and it’s density. Use the
following to determine both: mass = 96.408 g,
initial volume = 65.28 mL and final volume =
77.64 mL.
2. Mrs. Maki has found a sticky substance on one
of her desks and she wants to determine what it
is. If the substance weighs 8.1834 g has a final
volume of 82.46 mL and an initial volume of
76.53 mL, what is the substance?
Density Calculations
 Using the D=m/v formula you can also
determine mass and volume.
 m = D x v where the units are grams (g)
 v = m/D    where the units are mL or cm3
Density Calculations Examples
    Corn syrup has a density of 1.38 g/cm3.
What volume would need to be measured
for a mass of corn syrup weighing 24.5 g?
    Silver has a density of 10.5 g/cm3. What
would the mass be of a silver sample with
a volume of 36.5 cm3?
Density Calculations Examples
 What would the volume be for a
copper sample weighing 94.8 g?
 What would the mass be for a
chromium sample with a volume of
23.1 mL?
Density Calculations on WB
1. What would the mass be for a bakelite
sample with a volume of 14.6 mL?
2. What would the volume be for an oak
sample weighing 21.1 g?
3. A mahogany desk weighs 1653.7 grams.
What is the volume of the desk?
4. What is the mass of a lead pipe that has a
volume of 258.9 cm3?
Moles
 The amount of a substance that contains
6.02 x 1023 representative particles of that
substance
 Abbreviated mol
 6.02 x 1023 is know as Avogadro’s Number
 1 dozen eggs = ?
 1 mole of eggs = 6.02 x 1023
Moles
 1 mole of hydrogen = 1.008 g
 1 mole of calcium = 40.078 g
 1 mole of sodium =
 1 mole of potassium =
 1 mole of iron =
Mole Example Problems
 Calculate the number of moles in a
25.0 g sample of calcium.
 Calculate the number of moles in a
52.3 g sample of phosphorus.
 Calculate the number of moles in a
78.9 g sample of helium.
Mole Problems on WB
1. Calculate the number of moles in a 75.6 g
sample of chlorine.
2. Calculate the number of moles in a 89.8 g
sample of scandium.
3. Calculate the number of moles in a 13.7 g
sample of manganese.
4. Calculate the number of moles in a 20.4 g
sample of silver.
Mole Problems on WB
5. Calculate the number of moles in a 444 g
6. Calculate the number of moles in a 151 g
sample of mercury.
7. Calculate the number of moles in a 95.6 g
sample of osmium.
8. Calculate the number of moles in a 302 g
sample of tungsten.
Molar Mass/Formula Mass
 The mass of a mole of any substance.
 To find the molar mass of a compound, you
simply add up the total atomic masses of all
the elements in the compound.
 For example: CH4
C = 1 x 12.0 = 12.0
H = 4 x 1.0 = 4.0
MM of CH4 = 16.0 g/mol
Molar Mass/Formula Mass
 KNO3
 NH4
 Zn3(PO3)2
Molar Mass/Formula Mass
Examples on WB
1.   C6H12O6
2.   Na2SO4
3.   KMnO4
4.   Pb(CO3)2
5.   (NH4)2SO3
Calculating Mass from Moles
 To determine mass from moles, simply find
the molar mass of the compound and
multiply that by moles given in the problem.
 Determine the mass of a 4.86 mol of
CaCO3.
Ca = 1 x 40.08 = 40.08        MM = 100.08
C = 1 x 12.0 = 12.0           4.86 x 100.08 =
O = 3 x 16.0 = 48.0           486.39 g
Calculating Mass from Moles
Examples
 Calculate the mass of 1.48 mol of
K2O
 Calculate the mass of 2.68 mol of
Ag2SO4
 Calculate the mass of 5.89 mol of
Fe(NO3)3
Calculating Mass from Moles on WB

1. Calculate the mass of 4.13 mol of
Na2S2O3
2. Calculate the mass of 1.68 mol of
CsClO4
3. Calculate the mass of 2.46 mol of
Zn3(PO4)2
4. Calculate the mass of 6.41 mol of
LiSCN
Percent Composition

%         = Mass of element in Cmd x 100
Composition     MM of compound

 C2H5OH
 Na2SO3
Percent Composition Examples
 C10H14O
 C3H7OH
 CsClO4
 Be3(AsO3)2
Percent Composition on WB
1.   Fe(NO3)3
2.   NaHCO3
3.   Cd3(C6H5O7)2
4.   Na2S2O3
Empirical Formulas
 The simplest whole-number ratio of
atoms in a compound.
 Rules for Determining Empirical
Formulas:
1. Obtain the mass of each element
present from the problem.
2. Determine the number of moles of each
type of element.
Empirical Formulas
3. Divide the number of moles of each
element by the smallest number of moles
out of all the elements. This number will be
a whole or a half number – nothing else!! If
all the number are whole numbers after this
division, these numbers are the subscripts in
the empirical formula. If there is even one
half number, go to step 4.
Empirical Formulas
4. Multiply the numbers you got in step 3
by 2 to make them all whole numbers.
YOU MUST MULTIPLY ALL THE
NUMBERS BY 2, NOT JUST THE
HALF NUMBER. This set of numbers
will be your subscripts in the empirical
formula.
Empirical Formula Examples:
 An oxide of aluminum is formed by the
reaction of 4.151 g of Al with 3.692 g of O.
Calculate the empirical formula for this
compound.
 A sample of lead arsenate contains 1.3813
g of lead, 0.00672 g of hydrogen, 0.4995 g
of arsenic and 0.4267 g of oxygen.
Determine the empirical formula.
Empirical Formulas on WB
1. A sample of phosphoric acid contains 0.3086
g of hydrogen, 3.161 g of phosphorus and
6.531 g of oxygen. Determine the empirical
formula of phosphoric acid.
2. A sample of para-dichlorobenzene contains
5.657 g of carbon, 0.3165 g of hydrogen and
5.566 g of chlorine. Determine the empirical
formula for this compound.
Empirical Formulas on WB
3. A 4.550 g sample of cobalt reacts with 5.475 g
of chlorine to form a compound. Determine the
empirical formula for this compound.
4. When a 0.3546 g sample of vanadium metal is
heated in air, it reacts with oxygen to achieve a
final mass of 0.6330 g. Calculate the empirical
formula of this vanadium oxide. (HINT:
Subtract the mass of the vanadium sample
from the final mass to get the mass of oxygen
in the compound.)
Empirical Formula Examples
5. Sevin, the commercial name for an
insecticide used to protect crops such as
cotton, vegetables and fruit is made
from carbamic acid. A chemist
analyzing a sample of carbamic acid
finds 0.8007 g of carbon, 0.9333 g of
nitrogen, 0.2016 g of hydrogen and
2.133 g of oxygen. Determine the
empirical formula for carbamic acid.
Empirical Formulas from %
Composition
 A compound has been analyzed and
found to have the following percent
composition: 66.75% copper, 10.84%
phosphorus and 22.41% oxygen.
Determine the empirical formula for this
compound.
Empirical Formulas from %
Composition on WB
1. What is the empirical formula of a
sample whose mass percent
composition is: 21.9% Mg; 27.8% P;
50.3% O?
2. Determine the empirical formula of a
compound that is 29.0% sodium,
40.5% sulfur, and 30.4 % oxygen by
weight.
Molecular Formulas
 The exact formula of a molecule, giving
the types of atoms and the number of
each type.
 Molecular Formula = (Empirical Formula)n
 n = molar mass/empirical formula mass
 n = will be a whole number
 Molar mass will be given in the problem
Molecular Formula Examples
 A white powder is analyzed and found
to have an empirical formula of P2O5.
The compound has a molar mass of
283.88 g. What is the molecular
formula?
 The empirical formula of a compound is
NO2. It has a molar mass of 92 g.
What is its molecular formula?
Molecular Formula Examples
 A compound used as an additive for
gasoline to help prevent engine knock
shows the following percent
composition: 71.65% chlorine, 24.27%
carbon and 4.07% hydrogen. The
molar mass is 98.96 g. Determine the
empirical and molecular formulas for
this compound.
Molecular Formula Problems on WB

1. The empirical formula of a compound is
CH2. Its molecular mass is 70.0 g/mol.
What is its molecular formula?
2. Caffeine is a compound containing 49.47
% carbon, 5.191% hydrogen, 28.86%
nitrogen and 16.48% oxygen. The molar
mass of caffeine is 194 g/mol. Determine
the molecular formula for caffeine.
Molecular Formula Problems on WB

3. The empirical formula of a compound is
CH2O and its molar mass is 150 g/mol.
What is its molecular formula?
4. A compound contains 12.8% carbon, 2.1%
hydrogen, and 85.1% bromine by mass.
Calculate the empirical formula and the
molecular formula of this compound given
that the molecular mass is 188.0 g/mol.
Molecular Formula Problems on WB

5. A compound is 64.9% C, 13.5% H,
and 21.6% O. Its molecular mass is
88.0 g/mol. What is its molecular
formula?

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