Chemistry chapter 3

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molecular mass

sum of the masses of the atoms making up the molecule (units amu) (molecular compounds only!)

Molecular mass H₂O

2+15.9994= 18

Molecular mass C₂H₄O₂

122+4+162= 24+4+32=60

formula mass

is the sum of atomic masses of all atoms in a formula unit of any compound, molecular or ionic (units amu)

Formula Mass NaCl

22.9+35.4=58.3

MgCO₃

24.3+12+16*3=84

Define mole

substance is the quantity of a given substance that contains as many molecules or formula units as the number of atoms in exactly 12g of C-12.
Like 12 eggs in a dozen

Avogadro's Number

6.022 x 1023 molecules) is the numerical value assigned to the unit, 1 mole

Calculate mass with moles

Formula mass/6.02 x 10²³

How many Li₂SO₄ units are in 3 moles of Li₂SO₄?

7+32+16*4= 103
3moles x 6.02 x 10²³ x 1/1= 1.81 x 10²⁴

How many Li atoms are in 3 moles of Li₂SO₄?

3 moles x 6.02 x 10²³ x 2/1= 3.61 x 10²⁴

How many S atoms in 3 moles of Li₂SO₄?

3 moles x 6.02 x 10²³ x 1/1= 1.81 x 10²⁴

How many O atoms in 3 moles of Li₂SO₄?

3 moles x 6.02 x 10²³ x 4/1= 7.22 x 10²⁴

Molar Mass

mass of one mole of that substance
mass/avogadros #

What is mass in grams of Ca?

40.1 g/mol CA divided by /6.02 x 10²³= 6.66 x 10⁻²³

moles to grams

mass/avogardo

Mole calculations: consider ethanol with a molar mass of 46.1g/mol. Prepare acetic acid from 10.0 g of ethanol

10.0g x 1/ 46.1 = .217g/mol
grams divided by mass
grams to moles

Analysis of a solution shows it contains 0.909 mol of H2O2 in 1.00L of solution. What is the mass of hydrogen peroxide in this volume of solution?
molar mass of H2O2 is 34.02 g/mol

.909 mol * 34.02 g H₂O₂/1 ml H₂O₂= 30.92g
mole multiplied by mass
moles to grams

HCN is a volatile colorless liquid. The compound is highly poisonous . How many molecules are there in 56mg HCN, the average toxic does?

Convert
56mg -->grams x 1000 56000
56000/mass-->56000/27.02
multiply by avogardo
56000/27.02 * 6.022 x 10²³
1.2 x 10²³

Percent Composition

Mass % A = mass A in the whole X 100%
whole mass

The molecular mass of NH4NO3 = 80.05; its molar mass = 80.05g/mol.
Hence

N= 28.02/ total
28.02/80.05= .35 x 100%= 35%

How many grams of nitrogen are there in a fertilizer containing 48.5g of NH4NO3?

NH4NO3 is 35.0% N
(Fraction N = 0.350
48.5 x .35= 16.975= 17.0 g

Empirical Formula

Determined from data about percent composition, tells only the smallest whole number ratio of atoms in a compound.

Molecular Formula

n= molecular mass/empirical formula mass
C₂H₄O= 44amu
divide by actual mass, 44 amu = 1

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