In: Chemistry
QUESTION 10 Calculate the change in pH if 10mL of 0.1M HCl is added to the buffer made by mixing 25 mL of 1.0 M CH3COOH and 25 mL of 0.5 M CH3COONa. For these types of questions where a strong acid or base is added to a buffer solution, first write out the chemical reaction. What will the HCl react with: CH3COOH or CH3COO−? Since HCl is an acid, it will react with CH3COO-. Because HCl is a strong acid, it will completely react with CH3COO- to form CH3COOH. One approach to these problems is to make an ICE table and to work in mols. Complete the table below. Once the table is complete use the equation for [H3O+] in question 15. Q16 ICE table.png
pH =Pka + log (salt/acid)
sodium acetate is a salt of weak acid and strong base
CH3COOH is weak acid
moles of sodium acetate= 0.5*25/1000 =0.0125 moles
moles of acetic acid = 1*25/1000 =0.025
Volume of mixture= 25+25 =50 ml
concentration of acetic acid in the mixture = 0.025*1000/50=0.5 M
Concentration of sodium acetate =0.0125*1000/50= 0.25
ka of acetic acid = 1.8*10-5
pKa= -log (1.8*10-5)=4.744
pH= 4.744 +log (0.25/0.5)=4.74-0.30= 4.44
when 10 ml of 0.1 HCl is added, moles of HCl consmed= 0.1*10/1000 =0.001 moles
the following reaction takes place CH3COONa+HCl----> CH3COOH+ NaCl
CH3COONa gets consumed and CH3COOH gets added. Limting reactant is HCl since is it is only 0.001 moles while CH3COONa= 0.0125 moles
CH3COONa remaining =0.0125-0.001 = 0.0115
CH3COOH formed= 0.025+0.001 =0.026
concentration after mixting
CH3COONa =0.0115/60
CH3COOH= 0.026/60
pH= 4.74+log (0.015/0.026)=4.74-0.23889= 4.50
Chane in pH= 4.5-4.4 =0.06