Tap water 0.5 M hydrochloric acid (HCl) 0.5 M acetic acid (CH_3COOH) 0.5 M ascor
ID: 483071 • Letter: T
Question
Tap water 0.5 M hydrochloric acid (HCl) 0.5 M acetic acid (CH_3COOH) 0.5 M ascorbic acid (H_2C_6H_6O_6) 0.5 M ammonium chloride (NH_4CI) 0.5 M phosphoric acid (H_3PO_4) Look at the list of species in the Procedure Section. Write out the reaction when you place each species in H_2O. If more than one reaction can occur, write out all of them. It may be helpful to figure out the structure of each molecule particularly for those molecules with more than one ignitable proton. Predict - using chemical intuition, your past exposure to acids, or your gut feeling - the relative pH of a 0.5 M solution of each species (i.e. rank the solutions from most acidic to least acidic). Do this without doing any calculations! Your predictions aren't right or wrong, they are simply your based on your knowledge and experience up to this point. Once you have made your predictions, look up the K_a value in your textbook or online. If more than one reaction can occur when the species is placed in water, write out each reaction and assign a K_a to each reaction. Calculate the pK_a for each acid. Calculate the theoretical pH of a 0.5 M solution of species 2-7. Comment on whether your calculations match your predictions of relative acidity in question 1.Explanation / Answer
Q1)Tap water . pH = around 7.0
HCl + H2O ------> H3O+ + Cl-
HCl , pH <1
CH3COOH + H2O ------> CH3COO- + H3O+
acetic acid pH around 3
Though this is polybasic acid the first ioniation is only important.
HAsc + H2O -----> Asc- + H3O+
ascorbic acid pH around 3
NH4Cl + H2O -----> NH4OH + H+ Cl-
ammonium chloride around 5,
Even phosphoric acid is a tribasic acid, for this also the first ionization is only important
H3PO4 + H2O -----> H2PO4- + H3O+
phosphoric acid pH around 2-3
Thus HCl is the strongest of all.
the ranking is then HCl > phosphoric acid > acetic acid > ascorbic acid > ammonium chloride > water
Q2)2) pH of o.5M HCl = - log (O.5)
= 0.30
3) pH of 0.5M acetic acid . The pKa of acetic acid is 4.74
Thus ppH = 1/2 pKa -1/2 log C
= (4.74/2 ) - 1/2 log 0.5
= 2.21
4) pH of ascobic acid , pka1 of ascorbic acid = 4.17
pH = 4.17 -1/2 log (0.5)
= 4.12
5) ammonium chloride. It is a salt of weak base ad strong acid. Its pH is given by
pH = 1/2 [pkw -pkb -logc]
= 1/2 [14 - 4.74 -log0.5]
= 4.78
6) phosphoric acid whose pka1 = 2.15
thus pH = 1/2 (2.15) - 1/2 log (0.5)
= 1.23
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