Equivalents in Gen Chem

[huang119]

---

Members don't see this ad.

If a weak acid for ex. H3CCO2H has 2 equuivalents of its conjugate base H3CCO2K it has a one-to- one ratio and creates a buffer. Please explain! Really have a hard time comprehending what an equivalent is.

Other examples are:
NH3 + 2 equivalents NH4Cl
-this is also a buffer as well because of a one to one ratio.

This is what i was thinking. Ex: NH3 would require 1 equivalent of its conjugate pair to make a buffer. Hence it would need 1 equivalent and not 2 but that is wrong.

 

[Chrisz]

You sure it is stated this way. Monoprotic weak acids only need one equivalent base to make 1:1 ratio. Which book u using? 2 equivalent would make it 2:1 base to acid ratio. You sure you did not read it wrong. Equivalents just means # of equivalent parts to some reference part. For example, one mole of acetic acid only has one mole of protons, so it needs one mole of acetate. One mole of acetate is called one equivalent part to 1 part of acetic acid. This is the proper definition. If u have 3 Moles of h3po4, how many equivalent of oh- needed for neutralization. This would be 3 equivalents. my book use it this way

 

[Teleologist]

For example, one mole of acetic acid only has one mole of protons, so it needs one mole of acetate. One mole of acetate is called one equivalent part to 1 part of acetic acid.

You mean 1 mole of ionizable protons.

 

[Chrisz]

if we have one mole of OH-, how many equivalents of Ca2+ to fully precipiate, we need 0.5 mols Ca2+ for 1 mol of OH-, so 0.5molCa2+/1molOH-=0.5Eq
We have 1 mol H3PO4, How many equivalents of OH- to neutralize? we need 3molOH- for 1mol H3PO4, 3molOH/1Mol H3PO4=3Eq

 

[Chrisz]

You mean 1 mole of ionizable protons.

ignore about the proton part, just 1 mole of acetic acid needs 1 mol acetate fof1:1 buffer, so acetate=1Eq just wrote it down too fast, i did not know i write it down

 

[Entadus]

If a weak acid for ex. H3CCO2H has 2 equuivalents of its conjugate base H3CCO2K it has a one-to- one ratio and creates a buffer. Please explain! Really have a hard time comprehending what an equivalent is.

Other examples are:
NH3 + 2 equivalents NH4Cl
-this is also a buffer as well because of a one to one ratio.

This is what i was thinking. Ex: NH3 would require 1 equivalent of its conjugate pair to make a buffer. Hence it would need 1 equivalent and not 2 but that is wrong.

1 equivalent means things are in a 1:1 mole ratio. You're right that you need 1:1 acid:base ratio for a buffer at pH=pKa .... however the ratio may deviate away from this. More base means the pH will be higher than the pKa. More acid would mean pH is lower.

So a 2 equivalents NH4+ (weak acid) with his conjugate base NH3 would in fact make a buffer with a pH lower than the pKa of ammonium ion. (More acidic)

The equation we are using of course is the Henderson-Hasselbach:
pH = pKa + log([A-/[HA])

which is derived from

Ka = products/reactants = [H3O+][A-]/[HA]