Moles & Conversions

Moles & Conversions

2.3, 3.3, & 9.1

Atomic Mass&

Formula Mass

Atomic Mass is the mass of one atom and is listed on the periodic table with units of amu.

Atomic MassSamples: H Li O F Al

Masses:

Formula Mass is the sum of the atomic masses of all atoms in a compound with units of amu.

Formula Mass

Samples: H2O LiOH OF2

AlF3

Masses:

Mole,Avagadro’s Number

&Molar Mass

Mole In chemistry, we need a way to

count atoms and other small particles. We use something called the mole for this. It is a counting number like: 1 pair = 2 1 dozen = 12

Mole 1 mole = 6.022 x 1023 particles (particles may be atoms, molecules, or formula units)

Avagadro’s Number To find the number of particles in a mole, scientists used the number of atoms in 12 g of C-12 as a standard. In that sample, there are 6.022 x 1023 C-12 atoms.

Avagadro’s Number That number of C-12 atoms in 12 g is called Avagadro’s number.

Avagadro’s Number Avagadro’s number is defined as the number of particles in exactly one mole of a pure substance which is always 6.022 x 1023.

What’s in a mole? 6.022 x 1023 atoms per mole (of

atoms) 6.022 x 1023 molecules per mole

(of molecules) 6.022 x 1023 formula units per

mole (of formula units)

What’s in a mole? More than 6.022 x 1023 atoms: 1

mole CCl4 molecules is (5 × 6.022 x 1023) atoms

More than 6.022 x 1023 atoms: 1 mole NaCl formula units is (2 × 6.022 x 1023) atoms.

Molar Mass

is the mass of one mole of a compound with units of grams.

Molar Mass

The atomic mass on the periodic table can also be used for the molar mass of an element - switch amu with g.

Molar Mass 1 mole H atoms = _ atoms = _ g 1 mole N atoms = _ atoms = _ g 1 mole S atoms = _ atoms = _ g

Molar Mass Why are there different masses? atoms have different masses just like 12 eggs have different mass than 12 grapefruit

this is due to the varying numbers of protons, neutrons, & electrons

Mole Conversions will use dimensional analysis to

solve problems. allow us to convert between moles,

particles (atoms, molecules, etc.), mass, and volume.

use the molar masses and/or volumes for solving problems.

Mole Conversions If there are 16 g of S, how many moles are there?

If there are 2 moles of N atoms, what would the mass be?

Mole Conversions Find the mass in grams of 3.50 moles of Cu.

If you have 11.9 g of Al, how many moles do you have?

Mole Conversions How many moles of Ag are in 3.01 x 1023 atoms?

What is the mass in grams of 1.20 x 108 atoms of Cu?

Mole Conversions

Molar volume the volume of 1 mole of gas at STP equals 22.4 liters

Mole Conversions A chemical reaction makes 0.82 moles of O2 gas. What is the volume at STP?

A room has a volume of 4000. L. How many moles of air would that be (STP)?

Mole Conversions A 1.0 L flask of CO2 (@ STP)

contains how many CO2 molecules?

Percent Composition

7.3

Percent CompositionPercent Composition is the % by mass of an element in a

compound molar mass gives 100% of compounds

mass because it accounts for each element present in the compound

to get % composition, take [(mass of the element)/(molar mass of compound)] * 100%

Percent Composition

Percent CompositionExamples… Find the percent composition of

Cu2S.

Find the percent composition of PbCl2.

Percent Composition

Percent CompositionExamples… Find the percent composition of

Ba(NO3)2.

Find the percent composition of H in (NH4)2CO3.

Percent Composition

Percent CompositionExamples… Magnesium hydroxide is

54.87% O by mass. How many grams of O would be in 175 g of the compound?

Percent Composition

Percent CompositionExamples… Magnesium hydroxide is

54.87% O by mass. How many grams of O would be in 175 g of the compound? How many moles of O is that?

Empirical & Molecular Formulas

7.4

Empirical Formulas

Empirical Formulas consist of the elements’

symbols and the subscripts that show the smallest whole-number ratio of the atoms

Empirical FormulasFinding empirical formulas: Convert the % composition data to

grams. (HINT: If not given grams, use 100 grams of compound to start.)

Convert the composition in grams to moles by using molar masses of the elements. (This gives a mole ratio.)

Use the numbers from mole ratio to get the smallest possible whole number ratio.

Determining Empirical FormulasEx 1) A compound has 63.52%

Fe and 36.48% S. Find the simplest formula.

1. Comp. by mass: 2. Comp. in moles: 3. Smallest whole # ratio: 4. Simplest formula:

Determining Empirical FormulasEx 2) A 10.150 g substance sample

contains only P & O. After analysis, it is determined that there is 5.717 g O. What is the simplest formula of the compound?

1. Comp. by mass: 2. Comp. in moles: 3. Smallest whole # ratio: 4. Simplest formula:

Determining Empirical FormulasEx 3) A compound has 26.56%

potassium, 35.41% chromium, and 38.03% oxygen. Find the simplest formula for the compound.

1. Comp. by mass: 2. Comp. in moles: 3. Smallest whole # ratio: 4. Simplest formula:

Determining Empirical FormulasEx 4) A compound has 53.70%

Fe and 46.30% S. Find the simplest formula.

1. Comp. by mass: 2. Comp. in moles: 3. Smallest whole # ratio: 4. Simplest formula :

Molecular Formulas We’ve now seen how to

calculate empirical formulas, but that’s not always going to give the actual formula of a compound. (ex: AlCl3, Al2Cl6, Al3Cl9, etc…)

Determining Molecular Formulas What we can determine is that

there is a relationship between the simplest (empirical) formula and molecular formula. It is (simplest formula)x =

molecular formula. x is a whole number

Determining Molecular Formulas To determine the correct

formula, we need the formula mass. This will be related to the molecular formula mass in the same way as the formulas were related. (simplest formula mass)x =

molec. formula mass

Determining Molecular FormulasEx 1) The simplest formula of a

compound was determined to be P2O5. The formula mass was determined to be 283.889 amu. What is the molecular formula?

Find simplest mass: Find mass relationship: Find formula relationship:

Determining Molecular FormulasEx 2) Determine the molecular

formula of a compound with an empirical formula of CH and a formula mass of 78.110 amu.

Find simplest mass: Find mass relationship: Find formula relationship:

Determining Molecular FormulasEx 3) A compound with formula mass of

42.08 amu is found to have a composition by mass of 85.64% C and 14.36% H. Find the molecular formula.

Comp. by mass: Comp. in moles: Smallest whole # ratio: Simplest formula: Find simplest mass: Find mass relationship: Find formula relationship: