Midterm 1: pH, buffers, Collig. Props., etc.

Question 1

What indicator is used for pH of about 3.5?

Answer 1

Methyl Orange

Question 2

What indicator is used for pH of about 7?

Answer 2

Bromophenol blue

Question 3

What indicator is used for basic pHs of about 10?

Answer 3

phenolphthalein

Question 4

What indicator is used for pH of around 5?

Answer 4

methyl red

Question 5

Where does a buffer stabilize pH in relation to its pKa?

Answer 5

It stabilizes around pH value of its pKa +/- 1.

Question 6

What is a buffer’s “acid capacity”?

Answer 6

the moles of H+ that must be added to 1 L of the buffer to cause a pH drop of 1 point

Question 7

What is the definition of a buffer’s “base capacity”?

Answer 7

moles of OH ions that must be added to 1 L of the buffer to raise its pH by 1

Question 8

What does buffer capacit depend on and how?

Answer 8

Depends on the concentration and ratio of the salt and acid.

more salt = more acid capacity

1:1 ratio = equal acid/base capacity

more acid = more base capacity

Question 9

What must always be accounted for when calculating the pH of very dilute acid or base solutions?

Why?

Answer 9

the H+ and OH- content of water

because without it, a very dilute acid may appear to be pH > 7 and a very dilute base may appear to be pH < 7

Question 10

What is a hydrogen bond?

Is it a true bond?

Answer 10

an electrostatic interaction between polar molecules that occurs when a hydrogen atom bound to highly electronegative atom of N, O or F is attracted to some other nearby highly electronegative atom

• is not a true bond, but rather is a strong dipole-dipole interaction

Question 11

How is degree of dissociation calculated?

Answer 11

α = [H+]/C

Question 12

How is pH determined if concentration and pKa are known?

Answer 12

pH = (pKa - log C)/2

Question 13

How can pH be determined with Ka and concentration?

Answer 13

[H+] = √(Ka * C)

Question 14

What is the formula for osmotic pressure?

Answer 14

π = Osmolarity x R x T

pi is osmotic pressure

osmolarity is moles dissociable particles per liter

R is constant 0.31 (KPa*dm³)/(mol*K)

T is temp in kelvin

Question 15

What is the osmolarity of blood?

Answer 15

0.3

Question 16

What are the equations for boiling point elevation and freezing point depression of water?

Answer 16

ΔT_fp = Osmolarity * 1.86°C

ΔT_bp = Osmolarity * 0.512°C

(these temperatures are constants for water, other constants exist for other solvents)

(osmolarity is determined by moles of soluble particles per liter, ie 1 Molar NaCl = 2 Osmolar, b/c one mole each Na⁺ and Cl^-)