urgent chem help please! (1 Viewer)

[xxstef]

What is the pH of the solution which contains a mixture of 25mL of 0.25M HNO3 and 20mL of 0.35M KOH?

i've spent ages trying to work it out but i'm not getting anywhere
and the exam's tmr!

thanks

 

[Pwnage101]

work out how many moles of OH- there are

how many moles of H+ there are, and hence which one is in excess, and by how much [this is because k+ and no3- just dissolve in water)

then u know volume = 25+20 = 45 mL = .045L

then apply the pH/pOH formula

so for OH-:

n=cV = 0.35*0.02=0.007 moles

for H+:

n=cV=0.25*.025=.00625 moles

therefore we have 0.007-0.00625=0.00075 moles excess of OH-

new volume = 0.045L

therefore for OH-: c=n/v=0.00075/0.045==0.016667 mol/l

pOH= -log[OH-]=-log(0.016667)= 1.778

pH=14-pOH=14-1.778=12.22

 

[xxstef]

man i spent about an hr trying to do this one question. thank you so much!

 

[Pwnage101]

no worries, these types of questions are known as pH of solution questions, and if im not mistaken it was in a MC Q in last years HSC