Salt hydrolysis — pH of salt solutions (4 cases - SA - SB, SA - WB, WA - SB, WA - WB): Question bank

2.6.16 · D5 · Chemistry › Equilibrium › Salt hydrolysis — pH of salt solutions (4 cases - SA - SB,

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Definition "p" ka matlab aur key constants

Kisi bhi cheez ke aage chhota "p" matlab hai ==negative base-10 logarithm lo==. Toh $p X = - lo g_{10} X$ . Hamare constants par apply karte hain (neeche saare values reference temperature 25 °C par hain):

$K_{w} = [H^{+}] [OH^{-}] = 1 0^{- 14}$ paani ka self-ionization constant hai. Toh $p K_{w} = - lo g (1 0^{- 14}) = 14$ .
$K_{a}$ weak parent acid ka acid dissociation constant hai — bada $K_{a}$ = strong acid. $p K_{a} = - lo g K_{a}$ ; chhota $p K_{a}$ matlab zyada strong acid.
$K_{b}$ weak parent base ka base dissociation constant hai. $p K_{b} = - lo g K_{b}$ ; chhota $p K_{b}$ = stronger base.
$pH = - lo g [H^{+}]$ aur $pOH = - lo g [OH^{-}]$ , aur ye hamesha 25 °C par $p K_{w} = 14$ tak add hote hain.

Logs kyun use karte hain? Concentrations $1$ se lekar $1 0^{- 14}$ tak faili hoti hain — log us bade range ko friendly 0–14 pH scale mein squeeze kar deta hai.

Figure — Salt hydrolysis — pH of salt solutions (4 cases - SA - SB, SA - WB, WA - SB, WA - WB)

Formula pH formulas kahan se aate hain —

$h$ , $K_{h}$ , $C$ chain Ek hydrolyzing salt ko concentration $C$ par start karo. == $h$ == = degree of hydrolysis = woh fraction of ions jo react kar chuke hain ( $0 \leq h \leq 1$ ). Anion case ke liye mass balance ("ICE" bookkeeping) banao: $[CH_{3} COO^{-}] = C (1 - h), [CH_{3} COOH] = [OH^{-}] = C h$ KYA: har reacted ion ek CH₃COOH aur ek OH⁻ produce karta hai, isliye dono products $C h$ ke barabar hain. $C h$ kyun, $h$ kyun nahi: $h$ ek bare fraction hai; actual concentration nikaalane ke liye starting molarity $C$ se multiply karo. $K_{h}$ mein substitute karo: $K_{h} = \frac{( C h ) ( C h )}{C ( 1 - h )} = \frac{C h ^{2}}{1 - h} \approx h ≪ 1 C h^{2} \Rightarrow h = \frac{K _{h}}{C}$ $1 - h$ kyun drop karte hain: weak-parent ion ke liye almost kuch bhi react nahi karta, toh $1 - h \approx 1$ . Phir: $[OH^{-}] = C h = C \frac{K _{h}}{C} = K_{h} C = \frac{K _{w} C}{K _{a}} .$ $[H^{+}] = K_{w} / [OH^{-}] = K_{w} K_{a} / C$ mein convert karo aur $- lo g$ lo: $pH = \frac{1}{2} (p K_{w} + p K_{a} + lo g C) (WA/SB, basic) .$ SA/WB case iska mirror image hai ( $[H^{+}] = K_{w} C / K_{b}$ ), jo deta hai $pH = \frac{1}{2} (p K_{w} - p K_{b} - lo g C)$ . WA/WB case mein dono reactions ek saath chalte hain; $C$ factors cancel ho jaate hain, aur bacha rehta hai $[H^{+}] = K_{w} K_{a} / K_{b}$ aur $pH = \frac{1}{2} (p K_{w} + p K_{a} - p K_{b})$ .

True or false — justify

Ek strong acid ya strong base se aane wala ion phir bhi hydrolyze kar sakta hai agar solution concentrated enough ho.

False. Cl⁻, Na⁺, K⁺, NO₃⁻ mein kisi bhi concentration par H⁺ ya OH⁻ pakadne ki koi tendency nahi hoti — unke parents 100% dissociated hote hain, isliye reverse reaction ka essentially zero driving force hota hai. Concentration yeh change karta hai ki ek hydrolyzing ion kitna react karta hai, lekin yeh kisi non-hydrolyzing ion mein hydrolysis switch on nahi kar sakta.

NaCl solution ka pH har temperature par exactly 7 hota hai.

False. pH = 7 sirf 25 °C par hota hai, jahan

K_{w} = 1 0^{- 14}

. NaCl kabhi hydrolyze nahi karta, lekin

K_{w}

temperature ke saath badhta hai, isliye (maan lo) 50 °C par neutral pH 7 se neeche hota hai. NaCl neutral rehta hai, zaroori nahi ki pH 7 ho.

Acidic salt woh salt hai jisme acidic hydrogen atom hota hai.

False. Yahan "acidic salt" ka matlab hai ki solution acidic ho jaata hai (pH < 7) kyunki cation hydrolyze karta hai — jaise NH₄Cl mein koi acidic H nahi hota usual sense mein; uska NH₄⁺ paani ko H⁺ donate karta hai. Isse acid salts jaise NaHSO₄ ke saath mat confuse karo.

WA/SB salt ke liye, concentration

C

badhane se pH badhta hai.

True.

pH = \frac{1}{2} (p K_{w} + p K_{a} + lo g C)

se, bada

C

matlab

lo g C

kam negative hoga, toh pH badhega (zyada basic). Zyada anions matlab overall zyada OH⁻ produce hoga.

WA/WB salt ke liye, concentration double karne se pH change hota hai.

False. Uska formula

pH = \frac{1}{2} (p K_{w} + p K_{a} - p K_{b})

mein koi $C$ term nahi — cation se H⁺ aur anion se OH⁻ saath scale karte hain, toh unka ratio (aur isliye pH) concentration-independent hota hai.

Degree of hydrolysis

h

ek probability hai aur 0 aur 1 ke beech hona chahiye.

True.

h

woh fraction hai jitna ion paani ke saath react kar chuka hai, isliye

0 \leq h \leq 1

. Approximation

h ≪ 1

(jo

1 - h

term drop karne ke liye use ki jaati hai) bahut dilute ya strongly-hydrolyzing salts ke liye fail ho jaati hai, jahan

h

1 ke karib pahunch jaata hai.

Weak acid + weak base salt hamesha neutral hota hai.

False. Yeh sirf tab neutral hota hai jab

K_{a} = K_{b}

. Agar

K_{a} > K_{b}

toh cation ki acid strength jeet jaati hai → acidic; agar

K_{a} < K_{b}

toh anion ki base strength jeet jaati hai → basic. "Dono weak hain" ka matlab "exactly cancel ho jaayenge" nahi hota.

Spot the error

Ek student NH₄⁺ ke hydrolysis ke liye

K_{h} = K_{a} \times K_{b}

likhta hai. Galti dhundho.

Sahi relation hai

K_{h} = K_{w} / K_{b}

(weak base se aane wale cation ke liye). Product

K_{a} K_{b} = K_{w}

ek conjugate pair ko link karta hai, salt ke do alag parents ko nahi. NH₄⁺ ka

K_{a}

K_{w} / K_{b}

hai, aur woh

K_{h}

ke barabar hota hai.

Koi 0.1 M NH₄Cl ka pH

pH = \frac{1}{2} (p K_{w} - p K_{b} + lo g C)

se compute karta hai. Kya galat hai?

lo g C

par sign flip ho gaya hai. Sahi acidic-salt formula hai

pH = \frac{1}{2} (p K_{w} - p K_{b} - lo g C)

lo g 0.1 = - 1

ke saath, galat version wahan se 1 subtract karta hai jahan add karna chahiye tha, toh pH puri ek unit off aa jaata hai.

Ek student kehta hai: "CH₃COO⁻ paani ko H⁺ donate karke hydrolyze karta hai, jisse solution basic ho jaata hai." Unhe correct karo.

Acetate ek base hai — yeh paani se H⁺ accept karta hai (use wapas lekar CH₃COOH ban jaata hai), jisse excess OH⁻ peeche reh jaata hai. H⁺ donate karna ise acidic banata aur uske paas extra proton hona chahiye tha, jo hai nahi.

SA/WB pH derive karte waqt, ek student

C (1 - h)

term ko

C (1 - h) \approx 0

rakhta hai. Kahan galti hui?

Approximation hai

C (1 - h) \approx C

(kyunki

h ≪ 1

, toh

1 - h \approx 1

), na ki

\approx 0

. Ise zero set karna yeh kahega ki koi ammonium nahi bachi — jo bilkul bakwaas hai, kyunki almost kuch bhi react nahi kiya.

Ek student conclude karta hai ki Na₂SO₄ basic hai "kyunki yeh ek base se aaya." Diagnose karo.

Na₂SO₄ strong base NaOH aur strong acid H₂SO₄ se aata hai. Dono parents strong hain → koi bhi ion hydrolyze nahi karta → neutral. "Base se aaya" ka koi matlab nahi jab tak yeh check na karo ki woh base weak hai ya nahi.

Koi claim karta hai ki NH₄CN acidic hai kyunki NH₄⁺ H⁺ banata hai. Unhone kya bhool gaye?

Unhone CN⁻ ko ignore kiya, jiska parent HCN bahut weak acid hai (tiny

K_{a}

), isliye CN⁻ ek strong conjugate base hai jo H⁺ ko zyada strongly pakadta hai. Kyunki yahan

K_{b} (CN^{-}) > K_{a} (NH_{4}^{+})

hai, solution actually basic hai. WA/WB mein dono ko compare karna zaroori hai.

Why questions

Strong acid aur strong base ke salt ka neutral solution kyun deta hai jabki uske ions float kar rahe hain?

Kyunki koi bhi ion paani ke saath react nahi karta — dono parents poori tarah dissociated the, isliye ions ka koi "unfinished business" nahi. Paani ka apna autoionization

H_{2} O ⇌ H^{+} + O H^{-}

undisturbed rehta hai, jo

[H^{+}] = [OH^{-}]

maintain karta hai.

Hydrolysis constant

K_{h} = K_{w} / K_{b}

kyun likha jaata hai, na ki simply

K_{b}

K_{b}

us base ko describe karta hai jo paani se form hoti hai; uske conjugate acid ka hydrolysis reverse chemistry chalata hai.

K_{w}

(paani ka apna constant) ko

K_{b}

se divide karne par ion-vs-water reaction ka equilibrium constant milta hai, jo sahi taur par chhota hota hai kyunki parent weak tha.

Weaker parent acid apne salt ko zyada basic kyun banata hai?

Ek weaker acid apne proton ko zyada eagerly pakadta hai, toh uska conjugate base paani se H⁺ zyada strongly grab karta hai (bada

K_{h} = K_{w} / K_{a}

). Paani se zyada H⁺ remove hona → zyada leftover OH⁻ → zyada pH. Parent ki weakness = conjugate ki strength.

WA/WB pH formula mein concentration ki koi dependence kyun nahi hoti?

K_{h}

substitution mein dono products

C

ke saath scale karte hain jabki reactant bhi

C

ke saath scale karta hai, toh har

C

cancel ho jaata hai — algebraically

[H^{+}] = K_{w} K_{a} / K_{b}

mein koi

C

nahi bachta. Sirf

K_{a}

aur

K_{b}

ka balance (relative strengths) survive karta hai.

Hum

[H^{+}] = C h

kyun lete hain na ki

[H^{+}] = h

h

ek fraction hai (dimensionless); react hui ion ki actual amount hai (fraction) × (starting concentration) =

C h

, jo molarity ki units carry karta hai. Har reacted ion ek H⁺ (ya OH⁻) release karta hai, toh produced concentration

C h

hoti hai.

NH₄Cl ke liye

pH = - lo g C

(strong-acid shortcut) kyun use nahi kar sakte?

Woh shortcut maanta hai ki H⁺ mein complete dissociation hoti hai, lekin NH₄⁺ sirf partially hydrolyze karta hai (

h ≪ 1

). Uska zyada hissa NH₄⁺ ke roop mein rehta hai, isliye bahut kam H⁺ appear hota hai

C

se — pH

- lo g C

ki prediction se kahin zyada 7 ke karib hota hai.

Edge cases

Ek bahut dilute neutral salt, maan lo

1 0^{- 8}

M NaCl ka pH kya hoga?

Phir bhi essentially 7, 8 nahi. Salt kuch contribute nahi karta, aur itni kam "concentration" par paani ka apna

1 0^{- 7}

M H⁺ completely dominate karta hai. (Yeh wahi trap hai jaise

1 0^{- 8}

M strong acid — neutral salt add karke paani ko kabhi basic nahi bana sakte.)

Jab WA/SB salt ki concentration zero ki taraf jaati hai, toh uske pH ka kya hota hai?

Yeh upar se 7 ke karib jaata hai. Kam hydrolyzing anions ka matlab kam OH⁻ produce hoga, isliye chhota basic shift neutral ki taraf shrink hota hai. Formula ka

lo g C

term pH ko 7 ki taraf drive karta hai jab

C \to 0

(halanki paani ka floor ise 7 se neeche jaane se rokta hai).

Kya WA/WB "concentration-independent" formula bahut kam concentration par bhi sahi rehta hai?

Nahi. Derivation assume karti hai ki salt ka apna hydrolysis paani ke autoionization par dominate karta hai. Roughly

1 0^{- 6}

–

1 0^{- 7}

M se neeche paani se H⁺ aur OH⁻ (

1 0^{- 7}

M each) chhote salt contribution ko swamp kar dete hain, toh pH waapas 7 ki taraf drift karta hai aur neat

\frac{1}{2} (p K_{w} + p K_{a} - p K_{b})

result fail ho jaata hai.

WA/WB salt ke liye jab

K_{a} = K_{b}

ho, toh kya solution exactly neutral hota hai?

Effectively haan 25 °C par:

pH = \frac{1}{2} (p K_{w} + p K_{a} - p K_{b}) = \frac{1}{2} p K_{w} = 7

kyunki

K_{a}

aur

K_{b}

terms cancel ho jaate hain. Ammonium acetate (

K_{a} = K_{b} = 1.8 \times 1 0^{- 5}

) textbook example hai.

Temperature badhne par hydrolysis aur pH ka kya hota hai?

K_{w}

temperature ke saath badhta hai, toh hydrolysis constants (

K_{w} / K_{a}

K_{w} / K_{b}

) bhadte hain aur hydrolysis intensify hoti hai. Neutral pH khud 7 se neeche gir jaata hai, aur hydrolyzing salts zyada strongly shift karte hain — isliye saare pH values temperature ke saath quote karne chahiye.

Kya strong acid aur strong base ka salt kabhi apne ions ki wajah se non-neutral ho sakta hai?

Nahi. Definition se dono ions fully-dissociated strong parents se aate hain aur paani ko nahi chhute. Neutrality se koi bhi deviation temperature (

K_{w}

) ya impurity se aayega, kabhi ions se nahi.

Kya "

h ≪ 1

" approximation bahut dilute ya bahut weak-parent salts ke liye valid hai?

Hamesha nahi. Agar

C

bahut chhota hai ya

K_{h}

bada hai, toh

h

1 ke karib pahunch sakta hai, aur

1 - h

term drop karna bacha hua reactant overestimate kar deta hai. Tab tumhe poora

K_{h} = C h^{2} / (1 - h)

rakhna hoga aur

[H^{+}] = K_{h} C

use karne ki jagah quadratic solve karna hoga.

Recall Quick self-test

pKₐ mein "p" ka kya matlab hai? ::: Negative base-10 log lo: $p K_{a} = - lo g_{10} K_{a}$ . Acetate hydrolysis ke liye $K_{h}$ concentrations mein likho. ::: $K_{h} = \frac{[ CH _{3} COOH ] [ OH ^{-} ]}{[ CH _{3} COO ^{-} ]} = \frac{K _{w}}{K _{a}}$ . Acidic-salt (SA/WB) formula mein $lo g C$ ka sign kya hai? ::: Minus: $pH = \frac{1}{2} (p K_{w} - p K_{b} - lo g C)$ . WA/WB pH formula mein kaunsi ek quantity missing hai? ::: Concentration $C$ — yeh concentration-independent hai (jab tak bahut kam $C$ par paani ke apne ions dominate na karein). $h$ , $K_{h}$ , $C$ ko link karne wala relation (weak-parent ion)? ::: $h = K_{h} / C$ , toh $[H^{+}]$ ya $[OH^{-}] = C h = K_{h} C$ .