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 Mass Samples: 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: H 2 O LiOH OF 2 AlF 3 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 = x 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 x 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 x

What’s in a mole? x atoms per mole (of atoms) x molecules per mole (of molecules) x formula units per mole (of formula units)

What’s in a mole? More than x atoms: 1 mole CCl 4 molecules is (5 × x ) atoms More than x atoms: 1 mole NaCl formula units is (2 × x ) 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 atoms? What is the mass in grams of 1.20 x 10 8 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 O 2 gas. What is the volume at STP? A room has a volume of L. How many moles of air would that be (STP)?

Mole Conversions A1.0 L flask of CO 2 STP) contains how many CO 2 molecules?

Percent Composition 7.3

Percent 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 Examples… Find the percent composition of Cu 2 S. Find the percent composition of PbCl 2.

Percent Composition Examples… Find the percent composition of Ba(NO 3 ) 2. Find the percent composition of H in (NH 4 ) 2 CO 3.

Percent Composition Examples… Magnesium hydroxide is 54.87% O by mass. How many grams of O would be in 175 g of the compound?

Percent Composition Examples… 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 consist of the elements’ symbols and the subscripts that show the smallest whole- number ratio of the atoms

Empirical Formulas Finding 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 Formulas Ex 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 Formulas Ex 2) A g substance sample contains only P & O. After analysis, it is determined that there is 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 Formulas Ex 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 Formulas Ex 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: AlCl 3, Al 2 Cl 6, Al 3 Cl 9, 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 Formulas Ex 1) The simplest formula of a compound was determined to be P 2 O 5. The formula mass was determined to be amu. What is the molecular formula? Find simplest mass: Find mass relationship: Find formula relationship:

Determining Molecular Formulas Ex 2) Determine the molecular formula of a compound with an empirical formula of CH and a formula mass of amu. Find simplest mass: Find mass relationship: Find formula relationship:

Determining Molecular Formulas Ex 3) A compound with formula mass of 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: