joeyckn

I'm sure that all of you know the experiment to find Ca2+ in ppm in a sample of hard water by titrating with edta using Eriochrome Black T as an indicator.

During titration: H2Y2- (aq) + Ca2+ (aq) à CaY2- (aq) + 2 H+ (aq)

H2Y2- (aq) + Mg2+ (aq) à MgY2- (aq) + 2 H+ (aq)

At end point H2Y2- (aq) + MgIn- (aq) à MgY2- (aq) + HIn2- (aq) + H+

Wine-red sky-blue

1.The indicator reaction has to occur after the free Ca2+ or any free Mg2+ react. Why?

2.Hardness is traditionally expressed in mg CaCO3/L of water. Suppose that a water sample

you analyzed contained mainly Mg2+, how would the calculation of hardness be affected?

Explain.

3. Why [Mg-indicator]- is less stable than [Mg(edta)]2- and in turn less stable than [Mg-indicator]- ??

THX

What are criteria tha are usually used to judge whether a preparation method is good, besides yield and purity of the products?

For instance,benzoic acid form hydrolysis of benzamide..etc..organic preparation.

The melting point obtained by students is usually lower than the literature value. What are the reasons for this? What improvement can be made?