Chemistry and Biology P4 guesses?

[Jassim]

Any idea what topics are most probable to come in Bio and chem P4 this year? For those done with the pastpapers and theory completely, and who have nothing left do...

 

[princessnoor]

Its gonna be random and i think the paper will be basic not too hard but more chance of making silly mistakes loll

Can anyone plz help me solve this question
Nov11-43 Q3 e(iv)

 

[Alex10]

Its gonna be random and i think the paper will be basic not too hard but more chance of making silly mistakes loll

Can anyone plz help me solve this question
Nov11-43 Q3 e(iv)

Calculate the number of moles of acid left, so 0.0025-0.001 = 0.0015 (due to 1:1 mole ratio) and find its concentration by 0.0015/0.002 which gives 0.075 moldm^-3. One mole of NaOH reacts to form one mole of sodium ethanoate, so the concentration of the salt is 0.001/0.002 = 0.05 moldm^-3. Finally just apply the formula: pH = pKa + log(concentration of salt/concentration of acid)

 

[anonymous1122]

From one of the teachers

 

[princessnoor]

Calculate the number of moles of acid left, so 0.0025-0.001 = 0.0015 (due to 1:1 mole ratio) and find its concentration by 0.0015/0.002 which gives 0.075 moldm^-3. One mole of NaOH reacts to form one mole of sodium ethanoate, so the concentration of the salt is 0.001/0.002 = 0.05 moldm^-3. Finally just apply the formula: pH = pKa + log(concentration of salt/concentration of acid)

Where did the 0.002 come from and why is the moles of acid and salt divided by it?