Question H7a

[A Raab 1K]

H7a. Write a balanced chemical equation for each of the following reactions. (a) Calcium metal reacts with water to produce hydrogen gas and aqueous calcium hydroxide.
For calcium hydroxide, how would one know if the molecule is CaOH or Ca(OH)2? (and for other molecules that include hydroxide too)

[EvanWang]

Calcium is in the second group of the periodic table so calcium ions are Ca2+. Hydroxide ions have a -1 charge, so you need 2 OH to make the overall charge neutral. So Calcium hydroxide is Ca(OH)2.

[SnehinRajkumar1L]

Hydroxide has a -1 charge. Group 2 elements (including calcium), always have a +2 charge. So, for the charges to balance you would need 2 hydroxide molecules for the calcium ion. This applies to Group 1 elements where they always have a +1 charge. Elements in group 13, 14, 15, 16, and 17 have a +3, +4, -3, -2, and -1 charges respectively. Use this rule to determine the number of hydroxides in a compound.

[A Raab 1K]

Along that same theme, how would I know what to do for metal solids such as magnesium or for a nitrogen atmosphere? Magnesium is just Mg, whereas the nitrogen atmosphere is N2.

[Vanessa Chuang 4F]

Generally, if you're given a solid metal, it will stand by itself (ex: Mg, Cu, Ni, etc) unless the question states otherwise. However, for gases such as nitrogen and oxygen, those just happen to exist in the atmosphere as molecules (N2, O2, etc) which is why they need to be noted as so.

[Amir Bayat]

How do you know that hydroxide has a -1 charge?

Is this due to hydrogen being in the first row? Then what charge does oxygen have?

[MinuChoi]

Hydroxide has -1 charge because of oxidation numbers. O's oxidation number is -2 and H's oxidation number is -1.
You can also think of when water ionizes; it splits into a hydrogen ion (acidic) and hydroxide ion (basic). Hydrogen ions have charge +1 and water is neutral charge, so hydroxide has charge -1.