# A dozen. Molar Mass. Mass of atoms

## Full text

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Science 10

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is a number of objects.

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A dozen eggs, a dozen cars, and a dozen people are all 12 objects.

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But a dozen cars has a much greater mass than a dozen eggs because the mass of each car is much greater than the mass of each egg.

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An Au atom contains 79 p+ and 118 n°.

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An H atom contains one p+.

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The mass of an Au atom is about 197 times greater than the mass of an H atom.

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A dozen Au atoms have a mass that is about 197 times greater than the mass of a dozen H atoms.

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But atoms are very small…

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A gold atom has a mass of about 3 x 10-22 g or

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Because atoms are so small it is not practical to talk about individual atoms.

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Chemists talk about moles of atoms.

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Like a dozen, a mole is a number of atoms.

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One mole of atoms is about 6.02 x 1023

atoms.

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Just like 12 eggs is called a dozen eggs, 6.02!x!1023 atoms is a mole of atoms.

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6.02 x 1023 is known as Avogadro’s number.

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A mole of Au has 6.02 x 1023 atoms.

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A mole of Mg has 6.02 x 1023 atoms.

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The mole allows easy comparisons of amounts of atoms in the same way that the dozen allows easy comparisons of amounts of eggs.

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An oxygen atom has 8 p+ and 8 n°.

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Its atomic mass number is 8 + 8 = 16.

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6.02 x 1023 oxygen atoms have a mass of

16!grams.

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16 grams of oxygen atoms is a convenient amount of oxygen, whereas 16 atoms is impractical.

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is the mass of one mole of a substance.

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The atomic mass numbers of an element on the periodic table is the average mass, in grams, of one mole of that element’s atoms.

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What is the molar mass of Zn?

65.41 g/mol

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What is the molar mass of methane gas?

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Methane is CH4, meaning each molecule

has one C atom and 4 H atoms.

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Each C: 12.01 g/mol

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Each H: 1.01 g/mol

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Total: 12.01 g/mol + (4 x 1.01 g/mol) =

16.05 g/mol

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What is the molar mass of water? H2O:

H: 2 x 1.01 g/mol = 2.02 g/mol O: 1 x 16.00 g/mol = 16.00 g/mol

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What is the molar mass of iron (III) oxide? Fe2O3:

Fe: 2 x 55.85 g/mol = 111.70 g/mol O: 3 x 16.00 g/mol = 48.00 g/mol

M = 159.70 g/mol

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Page 108 practice questions

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One mole of Cu has a mass of 63.55 g.

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The molar mass (M) for Cu is 63.55 g/mol.

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63.55 g of Cu would be composed of 6.02!x!1023 atoms.

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127.10 g of Cu would be two moles of Cu, or 2 x 6.02 x 1023 = 1.204 x 1024 atoms.

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We can find the number of moles in a sample of any substance if we know (or can determine) the substance’s molar mass, and we know the mass of the sample.

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Find the number of moles in 2.00!g of helium.

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Find the molar mass:

He: 1 x 4.00 g/mol = 4.00 g/mol M = 4.00 g/mol 18

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What does 0.5 mol mean?

one mol = 6.02 x 1023 atoms

0.5 mol = 0.5 x 6.02 x 1023 atoms

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Find the number of moles in 6.00 g of strontium chloride.

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Find the molar mass: SrCl2: Sr: 1 x 87.62 g/mol = 87.62 g/mol Cl: 2 x 35.45 g/mol = 70.90 g/mol M = 158.52 g/mol 21

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… are a way of representing how accurate a measurement is.

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The digits 1 through 9 are always significant.

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Leading zeros are not significant.

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All other zeros are significant.

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In scientific notation, the digits before the “x!10” are significant.

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Exact numbers have unlimited significant digits.

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1.23

0.123

0.0123

103

120

12.0 25

9.0

15

0.43

5.0 x 104

10 26

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These are exact numbers:

15 students

\$5.25

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In multiplication or division calculations, the answer should be rounded to the least number of SD from the numbers used.

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Find the number of moles in 9.50!g of ethanol. C: 2 x 12.01 g/mol = 24.02 g/mol

### M = 46.08 g/mol

C2H5OH: H: 6 x 1.01 g/mol = 6.06 g/mol O: 1 x 16.00 g/mol = 16.00 g/mol 30

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Find the number of moles in 9.50!g of ethanol.

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Find the number of moles in 1.1!kg of gold (II) phosphate.

Au: 3 x 196.97 g/mol = 590.91 g/mol

### M = 780.85 g/mol

Au3(PO4)2:

P: 2 x 30.97 g/mol = 61.94 g/mol O: 8 x 16.00 g/mol = 128.00 g/mol

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Find the number of moles in 1.1!kg of gold (II) phosphate.

33

## =nM

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Find the mass of 0.205 mol of sodium carbonate.

Na: 2 x 22.99 g/mol = 45.98 g/mol

### M = 105.99 g/mol

Na2CO3:

C: 1 x 12.01 g/mol = 12.01 g/mol O: 3 x 16.00 g/mol = 48.00 g/mol

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Find the mass of 0.205 mol of sodium carbonate.

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Find the mass of 0.015 mol of calcium chloride.

Ca: 1 x 40.08 g/mol = 40.08 g/mol

### M = 110.98 g/mol

CaCl2:

Cl: 2 x 35.45 g/mol = 70.90 g/mol

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Find the mass of 0.015 mol of calcium chloride.

38

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