Gen Chem 3 Exam 2

Question 1

You have pH, how do you turn it into [H3O+] ?

Answer 1

Raise 10^ -pH

Question 2

You have pOH, how do you turn it into [OH-] ?

Answer 2

Raise 10^ -pOH

Question 3

You have pH, how do you turn it into [OH-] ?

Answer 3

Either:
First do 14 - pH to get pOH, then raise 10^ - pOH
or
First do 10^ -pH to get [H3O+], then do 10^-14 / [H3O+]

Question 4

You have pOH, how do you turn it into [H3O+] ?

Answer 4

Either:
First do 14 - pOH to get pH, then raise 10^ - pH
or
First do 10^ -pOH to get [OH-], then do 10^-14 / [OH-]

Question 5

Write a symbolic equation for a weak monoprotic base in water.

Answer 5

Either:
(YOH) – > (Y+) + (OH-)
or
(Y-) + (H2O) –> (HY) + (OH-)

You can of course use a different letter other than Y. And you can have different charges, as long as they are consistent. Y with a hydrogen should always have one higher charge than Y without a hydrogen. Also, I just put parenthesis here so that its easier to read; they aren’t there when you write it out on paper. And of course, you should use proper subscripts/superscripts when writing it out.

Question 6

Write a symbolic equation for a weak monoprotic acid in water

Answer 6

Either:
(HY) –> (H+) + (Y-)
or
(HY) + (H2O) –> (H3O+) + (Y-)

You can of course use a different letter other than Y. And you can have different charges, as long as they are consistent. Y with a hydrogen should always have one higher charge than Y without a hydrogen. Also, I just put parenthesis here so that its easier to read; they aren’t there when you write it out on paper. And of course, you should use proper subscripts/superscripts when writing it out.

Question 7

What is the name of the (H3O+) ion?

Answer 7

Hydronium ion

Question 8

What is the name of the (OH-) ion

Answer 8

Hydroxide ion

Question 9

What three things are always needed to work a weak acid in water problem?

Answer 9

A balanced reaction of a weak acid in water.
An ice table
Ka expression that uses molarities (not moles)

Question 10

What three things are always needed to work a weak base in water problem?

Answer 10

A balanced reaction of a weak base in water
An ice table
Kb expression that uses molarities (not moles)

Question 11

Sometimes its best to use molarity in your ICE table. Sometimes its best to use moles. How do you know what’s best?

Answer 11

If you are MIXING you always need moles. That usually happens when you are adding a strong acid or strong base to something.

If you are working a weak acid in water problem or weak base in water problem, the “K” expression NEEDS molarity. So it’s often convenient to do the whole ICE table in molarity if that’s allowed (i.e. if there’s no mixing going on in the problem)

Question 12

What is the Henderson Hasselbalch equation, and when is it used?

Answer 12

pH = pKa + log (Y- / HY) that’s pH = pKa + log (Weak base / Weak acid )

It’s used when you ONLY have a buffer.

Question 13

You have Ka. How do you get pKa?

Answer 13

Take the negative log of the Ka

Question 14

You have Kb. How do you get pKb

Answer 14

Take the negative log of the Kb

Question 15

You have pKa, how do you get Ka?

Answer 15

Raise 10^ - pKa

Question 16

You have pKb, how do you get Kb

Answer 16

Raise 10 ^ -pKb

Question 17

You have Ka, how do you get Kb?

Answer 17

Divide 10^ -14 by Ka

Question 18

You have Kb, how do you get Ka?

Answer 18

Divide 10^ -14 by Kb

Question 19

You have pKa, how do you get pKb?

Answer 19

Take 14 - pKa

Question 20

You have pKb, how do you get pKa?

Answer 20

Take 14 - pKb

Question 21

What are all the strong bases? There are eight of them. Give me ALL 8. You can use a periodic table if you want.

Answer 21

LiOH
NaOH
KOH
RbOH
CsOH
Ca(OH)2
Ba(OH)2
Sr(OH)2

Question 22

What are all the strong acids. There are 7 of them. Give me all 7. You can use the periodic table if you want.

Answer 22

HBr
HI
HCl
HClO3
HClO4
HNO3
H2SO4

Question 23

What are the cations (positively charged ions) that do not affect pH? There are 8 of them. Give me all 8. You can use a periodic table of you want.

Answer 23

Li+
Na+
K+
Rb+
Cs+
Ca (2+)
Ba (2+)
Sr (2+)

Question 24

What are the anions (negatively charged ions) that do not affect pH? There are six of them. You can use the periodic table if you want.

Answer 24

Br -
I -
Cl -
ClO3 -
ClO4 -
NO3 -

Question 25

Write the symbolic reaction for a strong acid added to a weak base.

Answer 25

Either:
(H3O+) + (Y-) —-> (H2O) + (HY)
or
(H+) + (Y-) —> (HY)

You can of course use a different letter other than Y. And you can have different charges, as long as they are consistent. Y with a hydrogen should always have one higher charge than Y without a hydrogen. Also, I just put parenthesis here so that its easier to read; they aren’t there when you write it out on paper. And of course, you should use proper subscripts/superscripts when writing it out.

*Note: This happens to be the reverse of the weak acid in water reaction!

Question 26

Write the symbolic reaction for a strong base added to a weak acid.

Answer 26

(OH-) + (HY) –> (H2O) + (Y-)

I would also accept
(OH-) + (Y+) – > (YOH) , but I personally think this version is much harder one to understand, so I don’t recommend it.

You can of course use a different letter other than Y. And you can have different charges, as long as they are consistent. For instance, Y with a hydrogen should always have one higher charge than Y without a hydrogen. Also, I just put parenthesis here so that its easier to read; they aren’t there when you write it out on paper. And of course, you should use proper subscripts/superscripts when writing it out.

*Note: This happens to be the reverse of the weak base in water reaction!

Question 27

How do you know if a pair of chemicals are a buffer?

Answer 27

They differ by a hydrogen (and possibly an inert ion like Na+) and both are weak.

Question 28

When do indicators change color?

Answer 28

The general rule we are using for this class is: Indicators are at their intermediate color in the range that is +/- 1 from the indicator’s pKa.

Question 29

Sometimes you should use “the denominator assumption”
Sometimes you should “completely use up the limiting reactant”

How do you know when to do each one?

Answer 29

The denominator assumption is used in a problem that just has a weak acid in water or just has a weak base in water. It happens when you have a K expression.

Using up the limiting reactant is done when there is a strong acid or a strong base being added to something.

Question 30

You are working a strong acid added to a weak base problem. You have used moles and used up the limiting reactant.

At the end of your ICE table, the chemicals that are present are: The weak base and the weak acid. What do you do next?

Answer 30

Use the Henderson-Hasselbalch equation to find pH, because all you have now is a buffer. If you are asked for something other than pH, use the square conversion table from test 1 to convert the pH into what you need.

Question 31

You are working a strong base added to a weak acid problem. You have used moles and used up the limiting reactant.

At the end of your ICE table, the chemicals that are present are: The weak acid and the weak base. What do you do next?

Answer 31

Use the Henderson-Hasselbalch equation to find pH, because all you have now is a buffer. If you are asked for something other than pH, use the square conversion table from test 1 to convert the pH into what you need.

Question 32

You are working a strong acid added to a weak base problem. You have used moles and used up the limiting reactant.

At the end of your ICE table, the chemicals that are present are: The strong acid and the weak acid. What do you do next?

Answer 32

Ignore the weak acid. Turn the strong acid (H3O+) moles into a strong acid molarity [H3O+]. Convert that molarity into whatever the question asked you to solve for using the square table from Exam 1.

Question 33

You are working a strong base added to a weak acid problem. You have used moles and used up the limiting reactant.

At the end of your ICE table, the chemicals that are present are: The strong base and the weak base. What do you do next?

Answer 33

Ignore the weak base. Turn the strong base (OH-) moles into a strong base molarity [OH-]. Convert that molarity into whatever the question asked you to solve for using the square table from Exam 1

Question 34

You are working a strong acid added to a weak base problem. You have used moles and used up the limiting reactant.

At the end of your ICE table, both reactants get used up and all you have is a weak acid: What do you do next?

Answer 34

Resist the urge to curse when you realize that you are only halfway done with the problem.

Convert those moles of weak acid (HX) into a molarity of weak acid [HX] using the total volume of the solution.
Write a brand new reaction of a weak acid in water, do a brand new ICE table, and set up a Ka expression.

Question 35

You are working a strong base added to a weak acid problem. You have used moles and used up the limiting reactant.

At the end of your ICE table, both reactants get used up and all you have is a weak base: What do you do next?

Answer 35

Resist the urge to curse when you realize that you are only halfway done with the problem.

Convert those moles of weak base (X- ) into a molarity of weak base [X-] using the total volume of the solution.
Write a brand new reaction of a weak base in water, do a brand new ICE table, and set up a Kb expression.

Question 36

Write the symbolic reaction for a strong acid added to a buffer.

Answer 36

Either:
(H3O+) + (Y-) —-> (H2O) + (HY)
or
(H+) + (Y-) —> (HY)

You can of course use a different letter other than Y. And you can have different charges, as long as they are consistent. Y with a hydrogen should always have one higher charge than Y without a hydrogen. Also, I just put parenthesis here so that its easier to read; they aren’t there when you write it out on paper. And of course, you should use proper subscripts/superscripts when writing it out.

*Note: This happens to be the reverse of the weak acid in water reaction!

Question 37

Write the symbolic reaction for a strong base added to a buffer.

Answer 37

(OH-) + (HY) –> (H2O) + (Y-)

I would also accept
(OH-) + (Y+) – > (YOH) , but I personally think this version is much harder one to understand, so I don’t recommend it.

You can of course use a different letter other than Y. And you can have different charges, as long as they are consistent. For instance, Y with a hydrogen should always have one higher charge than Y without a hydrogen. Also, I just put parenthesis here so that its easier to read; they aren’t there when you write it out on paper. And of course, you should use proper subscripts/superscripts when writing it out.

*Note: This happens to be the reverse of the weak base in water reaction!

Question 38

Just by looking at the chemical formula, how do you know that a chemical is a weak acid?

Answer 38

It is nitrogen with 4 things around it and one of them is a hydrogen (except for the chemical HNO3, which is a strong acid)

It has COOH at the end.

It begins with an H, and is not a strong acid.

Question 39

Just by looking at the chemical formula, how do you know that a chemical is a weak base?

Answer 39

It is a nitrogen with 3 things around it (except NO3- which is neutral, and HNO2 which is a weak acid)

It is the conjugate base of a weak acid. That is, it WOULD have been an acid, but it is missing its hydrogen.

Question 40

How do you write the reactions for a polyprotic acid problem?

Answer 40

Write the reaction of the original chemical plus water. Move a hydrogen over from it to the water. Write what results.

Take what results and react IT with water. Move a hydrogen over from it to the water. Write what results.

Repeat as many times as is needed.