Question about hydroxide

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Isabel Day 1D
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Joined: Fri Aug 09, 2019 12:15 am

Question about hydroxide

Postby Isabel Day 1D » Tue Oct 08, 2019 1:04 pm

Why is calcium hydroxide written as Ca(OH)2 and not just CaOH? This comes up in problem H.7

Hiba Alnajjar_2C
Posts: 108
Joined: Fri Aug 09, 2019 12:17 am

Re: Question about hydroxide

Postby Hiba Alnajjar_2C » Tue Oct 08, 2019 1:43 pm

Calcium hydroxide is written as Ca(OH)2 instead of Ca(OH) because it is a neutral compound rather than a charged one (meaning all of the charges add up to 0). In order for all of the charges to add up to 0, there have to be 2 OH, as OH's charge is -1 and Ca has a charge of +2. Therefore, in order to add up to a total charge of zero, you need 2 OH- to balance out the charge of 2+ in Ca.

(If you had Ca(OH), the total charge would be +1 rather than 0).

Posts: 104
Joined: Wed Sep 11, 2019 12:16 am

Re: Question about hydroxide

Postby 805394719 » Tue Oct 08, 2019 3:33 pm

Calcium has a +2 charge whereas hydroxide has a -1 charge. Because compounds are always neutral, there must be enough of both elements to cancel out the charges. This means that there should be a total of -2 charge for hydroxide to cancel out the +2 charge of calcium. Therefore, there must be 2 hydroxide atoms in this molecule, making the equation Ca(OH)2.

Sion Hwang 4D
Posts: 51
Joined: Wed Sep 18, 2019 12:21 am

Re: Question about hydroxide

Postby Sion Hwang 4D » Tue Oct 08, 2019 5:05 pm

If you look on the periodic table, Calcium is located on the 2nd column. This means that Ca has two valence electrons, causing it to have a positive 2 charge as an ion.
OH, hydroxide, has a negative 1 charge.

In order to balance the charge, there must be 2 OH ions for every Ca ion.

Hence, you have Ca2+ + OH- = Ca(OH)2

Robert Tran 1B
Posts: 118
Joined: Thu Jul 11, 2019 12:15 am

Re: Question about hydroxide

Postby Robert Tran 1B » Tue Oct 08, 2019 5:30 pm

The calcium ion has a 2+ charge, while the hydroxide ion has a 1- charge. Since calcium hydroxide does not have a charge, we need one calcium ion and two hydroxide ions to make the molecule neutral. Thus, the formula is Ca(OH)2.

Kylie Lim 4G
Posts: 110
Joined: Sat Aug 17, 2019 12:15 am

Re: Question about hydroxide

Postby Kylie Lim 4G » Tue Oct 08, 2019 5:38 pm

Calcium has 2 electrons that can be delocalized which gives it a charge of +2. The charge of hydroxide is -1, so to create a molecule with no overall charge you would need two hydroxide Ca(OH)2.

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