1) Fill in the blanks with either less or more. Consider two acids - HF and NH3.

Question

1) Fill in the blanks with either less or more.

Consider two acids - HF and NH3. Fluorine is________ electronegative than N and therefore HF will be__________ acidic than NH3.

2) Consider a 1.55 x 10-2M solution of H2CO3 where Ka1 = 4.5 x 10-7 and Ka2 = 4.7 x 10-11.

The concentration of CO32- is ______ x 10^_________

Note: Is there is not exponent then answer 10^0.

3).Buffer solutions with a pH of 10 can be prepared using sodium carbonate (Na2CO3) and sodium hydrogen carbonate (NaHCO3). What is the pH of a solution prepared by dissolving 10.0 g each of these two salts in enough water to make 0.250 L of solution? (pKb for Na2CO3 is 3.67)

4) Match the acid with it's correct pKa

H3PO4

H2PO4-

HPO42-

PO43-

options:

12.32

7.21

N/A

2.16

12.32

7.21

N/A

2.16

Explanation / Answer

1) Consider two acids - HF and NH3. Fluorine is more electronegative than N and therefore HF will be more acidic than NH3.

4) More the pKa value, lesser is the acididc strength and vice versa

Thus, H3PO4 = NA

H2PO4- = 2.16

HPO42- = 7.21

PO43- = 12.32

3) Molar mass of Na2CO3 = 106 g/mole

Thus, moles of Na2CO3 in 10 g of it = mass/molar mass = 10/106 = 0.0943

molar mass of NaHCO3 = 84 g/mole

Thus, moles of NaHCO3 in 10 g of it = mass/molar mass = 10/84 = 0.119

Now, as per Henderson's equation

pOH = pKb + log{[salt]/[base]} = 3.67 + log(0.0943/0.119) = 3.57

Thus, pH = 14 - pOH = 10.43

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