1. Iron(III) sulfide is an ionic compound. Therefore:
QUESTION:
Which of the following is the formula for iron(III) sulfide? A. FeS, B. Fe3S, C. Fe2S3, D. Fe3S2
Iron(III) sulfide is an ionic compound. Therefore:
Cation is iron(III), or Fe3+
Anion is sulfide, or S2-
Formula unit, the simplest combination of ions with a total charge of zero, consists of two cations and three anions x (+3) = x (-2) = -6
Cations: +3, +3, +3, +3, +3, …
Anions: -2, -2, -2, -2, -2, …
SCRIPT:
Which of the following is the formula for iron(III) sulfide? A. FeS, B. Fe3S, C. Fe2S3, D. Fe3S2
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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Iron three sulfide is an ionic compound…. We know it’s ionic because iron is a metallic element. In an ionic compound,
the cation is named first. Therefore….
the cation is iron three. The roman numeral three tells us that the charge of the cation is +3.
HIGHLIGHT
…. The anion is sulfide. By referring to a periodic table, we find that sulfur belongs to group six A. By now, you should know that
members of this group form monatomic ions with a charge of –2. Sulfide is S with a –2 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 two cations and three anions.
Two iron three cations… will have a total charge of +6. Two times +3 is +6.
… and three sulfide ions… will have have a total charge of –6. Three times –2 is –6.
Therefore, the formula should have subscripts to tell us that… in one formula unit…
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There are two cations…
And three anions….
The correct answer is C.
Note that we do not indicate the charges of the ions in the formula of a compound.
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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