化學 CHEMISTRY pH

Question

I would like to ask the following 3 questions:

what is the answer if you are asked to arrange the pH of NH4Cl, NaCl, HCl, H2SO4 and NaCH3COO in descending order (all are aqueous solution and 1M)?

which of the the four 1M solution of the following salts has the highest pH , NaNO3, Na2CO3, NH4Cl, NaHCO3?

How do you determine which of them have the highest pH, especially those NH4Cl, NaNO3, NaCl (not typical acid or alkali like HCl or NaOH)?

thanks a lot

Answer 1

NH4Cl - by hydrolysis gives a more acidic solutionNaCl - complete dissociation in water, pH = 7 neutralHCl - strong acid complete dissociation, slightly more acidic than H2SO4 (look in table printed in your textbook)NaCH3COO - by hydrolysis gives a more basic solutionTherefore, in descending order of pH:(highest ph, most basic) NaCH3COO > NaCl > NH4Cl > H2SO4 > HCl (lowest ph, most acidic)--------------------------------------------------------(for this question, split the two ions and add OH to the cation, and H to the anion)NH4Cl becomes NH4OH (which is NH3&water) + HCl = weak base + strong acid = gives a more acidic solutionNaCO3 becomes NaOH + HNO3 = strong base + strong acid = neutral solutionNa2CO3 becomes 2 NaOH + HCO3- = 2 mols strong base + 1 mol weak acid (very small Ka) = basic solutionNaHCO3 becomes NaOH + H2CO3 = strong base + weak acid (a bit larger Ka than HCO3-) = basic solutiontherefore, by comparing the last two, you can tell which is going to be more basic (have the highest pH)Na2CO3 will be the most basic solution because of the two moles of NaOH and the very little Ka of HCO3------------------------------------------------------it's actually very easy, just know your strong/weak acids and strong/weak bases(or you can memorize which are strong and weak electrolytes....but i prefer to do it this way)you split the cation and anion, add OH to the cation and H to the anion (except for NH4+ which becomes NH3)if you get:strong base + strong acid, the solution will be neutral (it is a strong electrolyte which completely dissociates in water)weak base + strong acid, the strong acid "wins", so the solution will be acidicstrong base + weak acid, the strong base "wins", so the solution will be basicweak base + weak acid cannot the determined just like this, you have to work out the hydrolysis of both ions and decide which you have more of, OH- or H+

Answer 2

At first, you should recognize the definition of pH, and the representation of pH value.
the simplest way to say that is : the more strong acid has a lower pH value.

1. the pH of NH4Cl, NaCl, HCl, H2SO4 and NaCH3COO in descending order (all are aqueous solution and 1M)?
H2SO4 < HCL < NH4Cl < NaCl < NaCH3COO
ACID: H2SO4 , HCL , NH4Cl
NEUTRAL: NaCl
ALKALI: NaCH3COO

2.. which of the the four 1M solution of the following salts has the highest pH ,
NaNO3, Na2CO3, NH4Cl, NaHCO3?
pH: Na2CO3 < NaHCO3 < NaNO3 < NH4Cl

Na2CO3 = NaCl + CO2
NaHCO3 = NaOH + H2CO3 strong alkali + weak acid
NaNO3 = NaOH + HNO3 strong alkali + strong acid
NH4Cl = NH3 + HCL weak alkali + strong acid

2006-08-20 17:19:43 補充：
sorry, there is something wrong, and see corrected one below:pH: Na2CO3 > NaHCO3 > NaNO3 > NH4Cl