1. Cobalt(II) chloride is an ionic compound. Therefore:
QUESTION:
Which of the following is the formula for cobalt(II) chloride? A. CoCl, B. CoCl2, C. CoCl2, D. Co2Cl2
Cobalt(II) chloride is an ionic compound. Therefore:
Cation is cobalt(II), or Co2+
Anion is chloride, or Cl-
Formula unit, the simplest combination of ions with a total charge of zero, consists of two cations and three anions x (+2) = x (-1) = -2
Cations: +2, +2, +2, +2, +2, …
Anions: -1, -1, -1, -1, -1, …
SCRIPT:
Which of the following is the formula for cobalt(II) chloride? A. CoCl, B. CoCl2, C. CoCl2, D. Co2Cl2
PAUSE
To name a compound given its formula, we need to first classify the compound, then apply the rule for that type of compound.
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Cobalt two sulfide is an ionic compound…. We know it’s ionic because cobalt is a metallic element. In an ionic compound,
the cation is named first. Therefore….
the cation is cobalt two. The roman numeral two tells us that the charge of the cation is +2.
HIGHLIGHT
…. The anion is chloride. By referring to a periodic table, we find that chlorine is a halogen… it belongs to group seven A. By now, you should know that
members of this group form monatomic ions with a charge of –1. Chloride is Cl with a –1 charge.
To write the formula for an ionic compound, we determine the simplest combination of ions with a total charge of zero. In this case,
the simplest combination consists of one cation and two anions.
One cobalt two cation… will have a charge of +2. One times +2 is +2.
… and two sulfide ions… will have have a total charge of –2. Two times –1 is –2.
Therefore, the formula should have subscripts to tell us that… in one formula unit…
CLICK CLICK
There is one cation…
And two anions….
The correct answer is B.
Note that we do not indicate the charges of the ions in the formula of a compound.
A subscript of one is also not written. When a subscript is not written, it is implied to be one.
CLICK PAUSE END RECORDING

2. Video ID: © 2008, Project VALUE (Video Assessment Library for Undergraduate Education), Department of Physical Sciences Nicholls State University Author: Glenn V. Lo Narrator: Funded by Louisiana Board of Regents Contract No. LA-DL-SELECT-13-07/08