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Weak and strong acid identification (and many other questions)

Submitted by genchem on Mon, 03/03/2008 - 17:49

Can I get someone to check my answers for me, and tell me how I did, thanks

genchem

Which one of the following is a strong acid?
1) H2CO3 
2) H2SO3 
3) CH3COOH
4) H2SO4 
5) H3PO4 

Answer: H2SO4

Question 2

The substance (CH3CH2)2NH is considered 1) a strong acid.
2) a weak acid.
3) a neutral compound.
4) a strong base.
5) a weak base.

Answer: a nuetral compound


Question 3

Which of the following pairs has the stronger acid listed first? 
1) H2AsO3, H2AsO4 
2) H2S, HCl
3) HI, HBr
4) HClO, HClO3 
5) H2SO3 , H2SO4 

Answer: HI, HBr

Question 4

Which of the following acids has the lowest pH?
0.1 M HBO, pKa = 2.43
0.1 M HA, pKa = 4.55
0.1 M HMO, pKa = 8.23
0.1 M HST, pKa = 11.89
pure water
1) HMO
2) pure water
3) HBO
4) HST
5) HA 

Answer: pure water

Question 5

What is the pH of a 0.050 M LiOH solution?
1) 12.70
2) 10-5 M 
4) 8.23 ´ 10-12 M 
5) 1.22 ´ 10-13 M 

Answer: >10-5 M

Question 7

Which one of the following pairs is not a conjugate acid-base pair?
1) HCl/H+ 
2) H2O/OH- 
3) H2O2/HO2- 
4) OH-/O2- 
5) H2PO4-/HPO42- 

Answer: HCl/H+

Question 8

A student adds 0.1 mol of oxalic acid and 0.1 mol of sodium dihydrogen phosphate to enough water to make 1.0 L of solution. The following equilibrium is established with the concentrations of the products greater than the concentrations of the reactants. Which of the statements about the equilibrium system is correct?
H2C2O4(aq) + H2PO4-(aq) « HC2O4-(aq) + H3PO4(aq)

1) The hydrogen oxalate anion, HC2O4-, is a stronger base than the dihydrogen phosphate anion, H2PO4-.
2) Water is a stronger acid than either oxalic or phosphoric acids.
3) Oxalic acid is a weaker acid than phosphoric acid.
4) Phosphoric acid is a weaker acid than oxalic acid.
5) The dihydrogen phosphate anion, H2PO4-, is a stronger acid than oxalic acid.

Answer: Phosphoric acid is a weaker acid than oxalic acid

Question 9

Which of the following aqueous liquids is the most acidic?
1) 0.1 M Pb(NO3)2, Ka = 3 x 10-8 
2) pure water
3) 0.1 M Al(NO3)3, Ka = 1 x 10-5 
4) 0.1 M Ni(NO3)2, Ka = 1 x 10-10 
5) 0.1 M Be(NO3)2, Ka = 4 x 10-6 

Answer: 0.1 M Be(NO3)2, Ka = 4 x 10-6

Question 10

If a strong acid such as HCl is diluted sufficiently with water, the pH will be higher than 7.
1) True
2) False

Answer: False


Here are the ones where you've got a problem:
2) (CH3CH2)2NH is basicly an NH3 molecule where two of the H atoms have been replaced by hydrocarbon chains.  Since NH3 is a base, this compound will be basic as well.

4) The larger the pKa the smaller the Ka value  (pKa = -log Ka).  The larger the Ka value, the stronger the acid.

5) LiOH is a base, so the pH of the solution MUST be greater than 7.  Since LiOH is a strong base and ionizes completely into Li+ and OH-, the concentration of the OH- ion will be .05 M.
Find the pOH (-log [OH-] or in this case -log(.05) ). 

Then remember than pH  +  pOH = 14, therefore  pH  = 14 -pOH

6)  You can do 6 in the same way that you did 5.
        Find the pH = - log [H3o+]  = - log (8.23 x 10-2)  in this case
Then remember than pH  +  pOH = 14, therefore  pOH  = 14 -pH
Final, find the [OH-] by taking the antilog (-pOH)

9) The larger the Ka value, the stronger the acid. 
(but remember to consider the exponents in judging the size of Ka.  For instance 8 x 10-4  is smaller than 2 x 10-2)


Hello ,
10.  pH tells the acidity or basicity if pH lies between 0-7 then . solution is acidic and if pH lies between 8 - 14 then, solution is basic. the solution with pH 7 are called as neutral.

Now what happens if we add water to a solution.
well the concentration of a solution can be checked by pH , e.g. if a solution of pH 2 is diluted with water so that volume becomes double, then pH is

pH = 2 means [H3O+]
    = 10 ^ -2 M.

after dilution to double the volume [H3O+] = 10^-2 M / 2
                                                        = 5 X 10 ^ -3 M
                                                        pH = - log (5X 10^-3)
                                                        = 3 -log 5
                                                        = 3 - 0.699  = 2.3

It means that pH increases on dilution.

I hope that helps.