Go over text Review It! Answers

By the way: if you missed the test last class, please make arrangements NOW with the sub as to when

you can come in and take that test…It needs to be taken before the next class period

Trade Emp/Mol Problems and Work them Rally Coach Style• Take all four problems and pass them to another

group• Work with your shoulder partners (you cannot work

on the same problem type that you work; don’t work the acid problem if you also wrote an acid problem)• Rally Coach – one partner works the problem while

their partner watches and coaches (if necessary); then the other partner works a second problem• Look over the two problems worked by your

teammates to make sure you know how to do their problems, too

Stoich Recap Videos

• Work with your face partner to answer the following questions (I will put who gets to answer the question in parenthesis)• If there are three people:• Can someone move to make pairs?• If not, just work together as a team of three

What is the first step of stoichiometry? (person with shortest hair answers)

• Get moles

What is the second step of stoichiometry? (person with most pets)

• Use coefficients to create a mole-to-mole ratio

What is the third step of stoichiometry? (tallest socks)• Find desired units

How do you know when to use stoichiometry? (darkest hair color)

• Stoichiometry is used when the chemicals you’re converting between are not the same• For example: stoich would be used here

• ? Moles Carbon = 10 grams Carbon dioxide• Stoich would NOT be used here

• ? Mole Carbon = 10 grams Carbon

What is a limiting reactant? (person wearing the most colors on their clothing)

• It’s the reactant that runs out first, stopping the reaction, and limiting the amount of products that can be produced

What is an excess reagent (aka excess reactant)? (person who didn’t answer the limiting reactant question)

• It’s the reactant that doesn’t run out, leaving extra when the reaction stops

In the video, I did some subtraction. What did the answer to that subtraction represent? (oldest)• It was the extra amount of product I could have

made with my excess reagent

How do you calculate percent yield? (youngest)• (Actual/Theoretical) * 100

How is the theoretical value obtained? (person with larger hand)

• Calculated using stoichiometry

If the percent yield is used in your dimensional analysis, where does it go? (longest hair)

• It goes in the mole-to-mole ratio WITH THE PRODUCT

Thank your partner!

• You need a periodic table, a calculator, and somewhere to work a problem

Combining Stoich and Emp/Mol Problems• Read the following question:

The molecular formula of a liquid hydrocarbon is to be determined by analyzing its combustion products.(a) The hydrocarbon burns completely, producing 7.2 grams of water and 7.2 liters of CO2 at standard conditions. What is the empirical formula of the hydrocarbon?

Keys to answering this question• Always try to write a balanced chemical reaction

(for any stoich question) this isn’t possible for this question• To calculate an empirical formula, you need to

know moles of each element• Keep the Law of Conservation of Matter in mind: • All the carbon in the hydrocarbon became the carbon in

carbon dioxide• All the hydrogen in the hydrocarbon became the

hydrogen in water

• Back to the question!

The molecular formula of a liquid hydrocarbon is to be determined by analyzing its combustion products.(a) The hydrocarbon burns completely, producing 7.2

grams of water and 7.2 liters of CO2 at standard conditions. What is the empirical formula of the hydrocarbon?

“at standard conditions” means the gas is at STP (standard temp and pressure) which means that you can use the fact that 1 mole of gas = 22.4 liters of gas

Work the problem. Get help from teammates if you’re stuck.

This is an old AP question, by the way (b) Calculate the mass in grams of O2 required for the complete combustion of the sample of the hydrocarbon described in (a).

Hint: since we don’t know the molecular formula, we can’t write a balanced reaction and use stoich…but we can figure out how much oxygen is in the water and how much oxygen is in the carbon dioxide.

• We can’t use stoich here because without the molecular formula, we can’t write a balanced equation. No equation = no stoich• Because of the law of conservation of mass, the mass of oxygen

in the products must be equal to the oxygen used to burn the hydrocarbon• There are a few ways to do the math. If you got the same answer,

your way worked!

(c) The density of the hydrocarbon is 2.37g/L. The temperature is 25C and the pressure is 1 atm, calculate the molecular weight of the hydrocarbon.

There is more than one way to work this problem…Use your equation sheet…

(d) What is the molecular formula of the hydrocarbon? Draw it and name it. [We will learn about organic chemistry when I return.]

• Insert answer to part d

(e) Write the balanced chemical equation with state signifiers…

Reminder: The molecular formula of a liquid hydrocarbon is to be determined by analyzing its combustion products.

2 C4H10(l) + 13 O2(g) 8 CO2(g) + 10 H2O(l)

Begin your homework

• Remember that chemistry is all about problem solving• You need to apply your understanding to a variety

of situations• If there is a chemical reaction and there is

information on one substance, stoichiometry can be used to determine all sorts of stuff• On the worksheet, complete problems 2, 3, 6