agenda do now
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Agenda
Welcome back!
The beginning…of ALL THE MATH!
Homework
PBJ procedure
Pages 1-3 of HW packet
Do Now
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.
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
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
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?
If you dissolve 5 moles of NaCl in enough water to produce 1 L of solution, what is the concentration?
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?
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.