For aqueous solution of how many of the following compounds/ mixture, does the pH remains constant even upon dilution? (Consider degree of ionisation/hydrolysis is negligible compared to 1)
$1) N H_{4} C l$
$2) N a_{2} C O_{3}$
3) A 1:2 molar ratio mixture of $N a_{2} S$ and $H C l$
4) A 5:2 molar ratio mixture of $N a O H$ and $H_{3} P O_{4}$
5) A 5:4 molar ratio mixture of $C H_{3} C O O N a$ and $H C l$
$6) N a H_{2} P O_{4}$
7) A 2:1 molar ratio mixture of $H C l$ and $N a H C O_{3}$
$8) C H_{3} C O O N H_{4}$
9) A 4:3 molar ratio mixture of $N H_{4} O H$ and $H C l$

Question

For aqueous solution of how many of the following compounds/ mixture, does the pH remains constant even upon dilution? (Consider degree of ionisation/hydrolysis is negligible compared to 1)
$1) N H_{4} C l$
$2) N a_{2} C O_{3}$
3) A 1:2 molar ratio mixture of $N a_{2} S$ and $H C l$
4) A 5:2 molar ratio mixture of $N a O H$ and $H_{3} P O_{4}$
5) A 5:4 molar ratio mixture of $C H_{3} C O O N a$ and $H C l$
$6) N a H_{2} P O_{4}$
7) A 2:1 molar ratio mixture of $H C l$ and $N a H C O_{3}$
$8) C H_{3} C O O N H_{4}$
9) A 4:3 molar ratio mixture of $N H_{4} O H$ and $H C l$

Accepted Answer

i) pH=(pKw−pKb−logC)ii) pH=(pKw+pKa2+logC)iii) Na2S+2HCl⇌2NaCl+H2S∴ pHWA=12(pKa1−logC)iv)  NaOH+H3PO4⇌NaNH2PO4+H2Oi52  f3022 NaOH+NaH2PO4⇌Na2HPO4+H2Oi32  f1022 NaOH+Na2HOP4⇌Na3PO4+H2Oi12  f0111∴ Buffer solution∴ pH=pKa3v) CH3COONa+HCl+CH3COOH+NaCli54  f1044∴  Acidic buffer∴  pH=pKa3vi) pH=pKa1+pKa22vii) HCl+NaHCO3⇌NaCl+H2CO3i21  f1011For a mixture of WA+SA1, major  1H+ ions will come from HCl.∴  pH=−log10[H+] HClviii) pH=12(pKw+pKa−pKb)ix) NH4OH+HCl⇌NH4Cl+H2Oi43  f1033∴ Basic bufferClearly, ph of solutions 4,5,6,8 and 9 is not affected by dilution.

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	$N a O H + H_{3} P O_{4} ⇌ N a N H_{2} P O_{4} + H_{2} O$
i	5	2
f	3	0	2	2
	$N a O H + N a H_{2} P O_{4} ⇌ N a_{2} H P O_{4} + H_{2} O$
i	3	2
f	1	0	2	2
	$N a O H + N a_{2} H O P_{4} ⇌ N a_{3} P O_{4} + H_{2} O$
i	1	2
f	0	1	1	1

	$C H_{3} C O O N a + H C l + C H_{3} C O O H + N a C l$
i	5	4
f	1	0	4	4