Question

1. The degree to which a weak base dissociates is given by the base-ionization constant, Kb. For the generic weak base, B B(aq)+H2O(l)⇌BH+(aq)+OH−(aq) this constant is given by Kb=[BH+][OH−][B] Strong bases will have a higher Kb value. Similarly, strong bases will have a higher percent ionization value. Percent ionization=[OH−] equilibrium[B] initial×100% Strong bases, for which Kb is very large, ionize completely (100%). For weak bases, the percent ionization changes with concentration. The more dilute the solution, the greater the percent ionization. Ammonia, NH3, is a weak base with a Kb value of 1.8×10−5.

partA What is the pH of a 0.245 M ammonia solution?

partB What is the percent ionization of ammonia at this concentration?

2. To understand the relation between the strength of an acid or a base and its pKa and pKb values. The degree to which a weak acid dissociates in solution is given by its acid-ionization constant, Ka. For the generic weak acid, HA, HA(aq)⇌A−(aq)+H+(aq) and the acid-ionization constant is given by Ka=[A−][H+][HA] Similarly, the degree to which a weak base reacts with H2O in solution is given by its base-ionization constant, Kb. For the generic weak base, B, B(aq)+H2O(l)⇌BH+(aq)+OH−(aq) and the base-ionization constant is given by Kb=[BH+][OH−][B]

Another way to express acid strength is by using pKa: pKa=−logKa Another way to express base strength is by using pKb: pKb=−logKb

partA: A new potential heart medicine, code-named X-281, is being tested by a pharmaceutical company, Pharma-pill. As a research technician at Pharma-pill, you are told that X-281 is a monoprotic weak acid, but because of security concerns, the actual chemical formula must remain top secret. The company is interested in the drug's Ka value because only the dissociated form of the chemical is active in preventing cholesterol buildup in arteries. To find the pKa of X-281, you prepare a 0.086 M test solution of X-281 at 25.0 ∘C. The pH of the solution is determined to be 2.40. What is the pKa of X-281? pKa=?

3. Classify each salt as acidic, basic, or neutral. (KBr, NaNO2, AlCl3)

4. Rank the following compounds in order of decreasing acid strength using periodic trends. strongest acid to weakest acid (MgH2, HI, HBr, H2Se)

Answer 1

1) Part a.

The equilibrium reaction is

............................................NH3 + H2O <==========> NH4⁺ + OH^-   

.moles at equilibrium.......(0.245 - x) .......................................x ...................x

We assume [OH^- ] = x when equilibrium is reached. Thus we can write the equilibrium expression

as.............................................K_b   = [ NH4⁺ ] [OH^- ] / [ NH3 ]

........................................................= x² / (0.245 - x )

since , x is insignificant relative to 0.245 , we can write

...................................................K_b = x² / 0.245

.............solving for x ,

......................................................x = SQRT { 0.245 x ( 1.8 x 10^-5 ) }

..........................................................= 2.1 x 10^-3 M

,.................................................&,x = [OH^- ]

...............................................p(OH) = -log(OH^- )

..........................................................= - {log (2.1 x 10^-3 ) }

..........................................................= 2.68

we know that

................................pH + p(OH ) = 14

......................................................pH = (14 - 2.68 )

............................................................ = 11.32

Part b.

Since, NH3(aq) is a weak base, Ostwald's dilution law is applicable for calculating its degree

of dissociation () . Accordingly,

.................................................... = SQRT ( K_b / C )........where C represents concentration

.............................................................................................of NH3 in moles / L ( ie M )

Substituting the given values .....  = SQRT{ ( 1.8 x 10^-5 ) / 0.245 }

.........................................................= 0.8571 x 10^-3

hence , % ionization ........................= (0.8571 x 10^-3 x 100)

..........................................................= 0.86%

2)

Using Ostwald's dilution law we calculate K_a  of X-281 which has been identified as a weak

monoprotic acid

K_a = C ²   

...... = 0.086 ( 3.98 x 10^-3 )²

.........= 1.36 x 10^-6

& pK_a = - log K_a  

.......... = -log (1.36 x 10^-6 )

...........= 5.86

========================================================================

Please post the other two questions separately as fresh questions. Glad to help.