A beaker with 190mL of an acetic acid buffer with a pH of 5.00 is sitting on a benchtop. The total molarity of acid and conjugate base in this buffer is 0.1M . A student adds 8.70mL of a 0.420M HCl solution to the beaker. How much will the pH change? The pKa of acetic acid is 4.76.
Express your answer numerically. Use a minus (-) sign if the pH has decreased.
2007-07-30 03:08:44 · 1 answers · asked by Anonymous in Science & Mathematics ➔ Chemistry
the pH will decrease since you add an acid
you have the formula
pH =pKa + log [base]/[acid ] [ ]=concentration
5=4.76+log[base]/[acid ]
log[base]/[acid ] =0.24
so [base]/[acid ] =1.738 and [base]=1.738[acid]
and the molarity of the acid is 0.1= 2.738[acid]
[acid]= 0.1/2.738=0.0365
concentration of base being .1-0.0365 =0.0635M
the number of moles of acid is 0.19*0.0365=0.006935M
you add to this 8.7mL= 0.0087L of 0.42m HCL =0.003654 mole of acid
and the mole of acid= 0.006935+0.003654= 0.007308
so the pH is pH =4.76+ log0.00635/0.007308 =4.76-0.06=4.7
final answer 4.7
the pH has dropped -0.3 unit
2007-07-30 03:44:16 · answer #1 · answered by maussy 7 · 0⤊ 0⤋