Review Topics Use the References to access important values if needed for this q

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Review Topics Use the References to access important values if needed for this question. Design a buffer that has a pH of 10.20 using one of the weak base/conjugate acid systems shown below. Weak Base Kb Conjugate Acid K.pKa CHINH2 4.2x CH,NH CoHsON5.9x10 C&H; O NH 1.7x10 7.77 C,H,N | 1.5x 109 CJI-NII+ 2.4x10 10.62 6.7x106 5.17 How many grams of the bromide salt of the conjugate acid must be combined with how many grams of the weak base, to produce 1.00 L of a buffer that is 1.00 M in the weak base? grams bromide salt of conjugate acid =[ ? grams weak base Submit Answer Retry Entire Group 7 more group attempts remaining

Explanation / Answer

Solution:-

pH of the buffer is 10.20

the range of the buffer is = pKa plus minus 1 ( pKa +- 1 )

so the appropriate buffer is First one CH3NH2 and CH3NH3+

Molarity of week base = 1.00 M

number of moles of week base = Molarity * volume in liters

= 1 M * 1 L = 1 mole

now we can calculate the mass of base

Mass of base = mole * molar mass = 1 mole * 31.1g/mol = 31.1 g

gram of week base = 31.1 g

according to handerson- Hasselbalch equation = pH = Pka + log [ salt/acid] -----------(1)

so from this we will calculate the molarity of salt , so put all the values in equation (1)

10.20 = 10.62 + log[ salt/ 1.00M ]

log[ salt / 1.00 M ] = -0.42

so concentration of salt = 10-0.42 M = 0.38 M

mole of salt = molarity * volume = 0.38 * 1.00 = 0.38 moles

mass of salt = mole * molar mass = 0.38 mole * 111.97 g/mole [ Molar mass of CH3NH3Br = 111.97 g/mol ]

= 0.38 mole * 111.97 g/mole = 42.55 g

gram of Bromide salt of conjugate acid = 42.55 g

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