Agenda

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The beginning…of ALL THE MATH!

Homework

PBJ procedure

Pages 1-3 of HW packet

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All the math

Molar Mass – the mass of one mole of any substance, reported in grams (gram atomic mass)

Molar mass – sum of the molar masses of the elements in the compound

Example: CH4

The molar mass of CH4=16.05 g/mole

Element # mol element/mol compound

Molar Mass Mass contribution

C 1 12.01 g/mol 12.01 g

H 4 1.01 g/mol 4.04 g

For example, the molar mass for the element, Al, is 27.01 g. This means that 1 mole of Al atoms has a mass of 27.01g.

In other words, 6.02 x 1023 Al atoms has a mass of 27.01 g.

Relative amounts of each element in a compound

Can be used as a conversion factor

What is the percent by mass of nitrogen in NH3?

MM of NH3: 17.04 g/mol

Mass contribution of N: 14.01 g/mol

(14.01g/mol)÷(17.04g/mol) * 100 = 82.22%

We use it to count, just like we use dozen or pair.

If we have: 1 dozen roses= 12 roses

If we have one mole of anything: We have 6.02x1023 of that thing

Avogadro’s number

A chemical formula showing the lowest whole # ratio of elements in a compound

Can be calculated from percent composition

Example: Find the empirical formula for a compound that is composed of 62.1% C, 13.8% H, and 24.1% N.

To determine formula:

Convert the masses of each element into moles.

Divide each mole value by the smallest mole value to get mole ratio

Write empirical formula using symbols, and use the whole-number ratio as subscript.

Non-whole numbers – multiply by 2 or 3 to make them whole

C 62.1g

H 13.8g

N 24.1 g

will be identical to or some multiple of the empirical formula.

Show the actual number of each kind of atom present

Example: C2H6=Molecular Formula

CH3=Empirical Formula

Example: CH4=Molecular Formulas

CH4=Empirical Formula

Steps

*Find empirical formula

*Calculate molar mass for empirical formula

*Divide MM molecular/MM empirical

*Multiply this factor by each of subscripts in empirical formula to give molecular formula

Chapter 10

Three steps:

List the knowns and the unknown.

Solve for the unknown.

Select the appropriate unit equality and make it into a conversion factor, with the unknown’s unit in the numerator and the known’s unit in the denominator.

Multiply the known quantity by the conversion factor.

Make sure to eliminate all but the unknown’s unit.

Evaluate - Does the result make sense?

We use moles to count particles like: atoms, molecules, formula units, or ions.

One mole of moles (the animal) would have the equivalent mass to 60x the Earth’s oceans.

How many atoms are there in 1 mole of sodium?

6.02x1023 atoms

How many formula units are there in 1 mole of KOH?

6.02x1023 formula units

How many molecules are there in one mole of water?

6.02x1023 molecules

Example: How many moles of water molecules are in 1.21 x 1024 water molecules?

Example: How many Cu atoms are in 2.54 moles of Cu atoms?

If you are given a certain mass of a substance, you can find out how many moles you have by using the molar mass.

Example: How many moles of carbon atoms are found in 24.02 g of carbon?

24.02g x (1mole) = 2.000 moles C

(12.01g)

One mole of any gas at STP occupies a volume of 22.4L.

S.T.P.-Standard Temperature and Pressure. The values are 0 ◦Celsius and 1 atmosphere of pressure.

Example: What is the volume of 2.55 moles of hydrogen gas @STP?

Mass1.00 mol

molar mass

molar mass

1.00 mol

MOLEParticles

6.02x1023particles

1.00 mol

1.00 mol

6.02x1023particles

Volume(gas at STP)

22.4 L

1 mol

1 mol

22.4 L

The calculation of quantities in chemical reactions

A balanced chemical equation becomes a recipe that we can use

2Na + Cl2 2NaCl

In the above equation, the coefficients tell you:

The relative number of particles

The relative number of moles

If and only if all substances are gaseous, the relative number of liters @STP

How many moles of chlorine gas are needed to react with excess sodium to produce 5.00 moles of sodium chloride?

A unit of concentration

Units are mole solute/liter solution

Conversion factor!

If you dissolve 5 moles of NaCl in enough water to produce 1 L of solution, what is the concentration?

How many grams of water can be produced when 64.00 grams of oxygen react with excess hydrogen?

2H2 (g) + O2(g) 2H2O (g)

Example: How many liters of oxygen @ S.T.P. are needed to completely react with 6.0 liters of hydrogen to produce water?

Measurement of efficiency of a reaction

A factory worker in a paint company follows a batch slip to prepare a 1500 gallon batch of paint. However, when the batch goes to the filling department only 1489 gallons are filled. What is the % yield?

As in the cookie recipe, if you run out of eggs (and can’t get more), but have the rest of the recipe, you can’t make any more cookies.

In chemistry we call the reactant that runs out first, the limiting reactant/reagent.

All other reactants that we have in excess we call excess reagents.

2H2 + O2 2H2O

If 40.00g of hydrogen react with 25.00 g of oxygen, what mass of water will be produced?