Question

7. Calculate the ratio of the molarities of HPO and H2PO ions required to achieve buffering at pH 7.00. For HaPOa, pKat 2.12, pKa2 7.21, and pKas 12.68. 8. At the stoichiometric point in the titration of 0.260 M CH NH2(aq) with 0.260 M HCI(aq) a) the pH is less than 7. b) [CH3NH3'] 0.260 M. c) the pH is 7.0. d) [CH3NH2] 0.130 M. e) the pH is greater than 7 f) 9. Sketch the titration curve for the titration of 10 mL 0.2M NaOH with 0.1 M HCl Indicate the initial pH and the pH at the stoichiometric point. a) b) What volume of HCl has been added at the stoichiometric point? Calculate the pH after the addition of 5 mL acid c)

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Answer 1

Q -7 ) H2PO4^– & HPO4^2- are in second dissociation of H3PO4

using Henderson equation

P^H = P^Ka2 + log [ HPO4^2-] / [ H2PO4^- ]

7 = 7.21 + log [ HPO4^2- ] / [ H2PO4^- ]

[ HPO4^2- ] / [ H₂PO4^- ] = 10^–0.21 = 0.62

Q- 7 ) at equivalence point acid - base reaction is complete and salt CH3NH3Cl is form whose concentration will be 0.13 M and due to hydrolysis of CH3NH3⁺ solution will be acidic so P^H will be less than 7

correct answer is option a

Q -8 ) ( a ) initially 0.2 M NaOH is present which is strong base so [ OH^- ] = 0.2

P^OH = — log ( 0.2 ) = 0.70
P^{H =} 14 — 0.70 = 13.3

P^{H _at} stoichiometric point will be 7.

( b ) volume of HCl = ( 0.2 M ) x ( 10 mL ) / ( 0.1 M ) = 20 mL

( c ) moles of OH^- remaining =( 2 - 0.5 ) x 10^{–3 =} 1.5 x 10^–3 mol

[ OH^— ] = ( 1.5 x 10^–3 mol ) /( 15 x 10^–3 litre ) = 0.1 M

P^{OH =} 1 , P^{H =} 14 —1 = 13

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