Organic Compounds: Hydrolysis, Oxidation & Structural Formulas

[Pseudo Statistic]

This question came up on one of our worksheets:
A fruity compount A (C2H4O2) is hydrolysed by distilled H2SO4 to give B and C (CH2O2). C reacts with Mg to yield H2 and compound D. B is oxidised to form methanoic acid. Give the structural formula and reaction. Name A, B, C and D.

The solution ends up giving the reaction:
2CH2O2 + Mg -> [HCO(O-)]2 Mg2+ (Drawn as an ester except the single bonded O is shown to be negatively charged) + H2

I just thought I'd ask... can someone tell me why we're allowed to show the charge here?
The reason I post this not in the homework section is because I want some pointers on where I can look to begin to understand this kind of stuff, rather than just getting help for answering the question.

Thank you for any replies.

 

[Gokul43201]

The solution ends up giving the reaction:
2CH2O2 + Mg -> [HCO(O-)]2 Mg2+ (Drawn as an ester except the single bonded O is shown to be negatively charged) + H2

I just thought I'd ask... can someone tell me why we're allowed to show the charge here?

[HCO(O-)]2 Mg2+ is not an ester; it is a salt of a carboxylic acid (or a carboxylate). And the charges are specified to indicate that it is a fairly ionic salt.