# Relation between number of equivalents of reactants

Consider the reaction $\ce{Sb2S3 + F2 -> SbF5 + SF6}$. If the milliequivalents of antimony, sulfur and fluorine are x, y and z respectively, then prove that $x+y=z$.

This is what I know: Number of equivalents of each reactant in the reaction is equal, and that number of equivalents is (mass of species) /(equivalent weight). The problem I'm facing is that $\ce{Sb}$ and $\ce{S}$ are in the same compound, so I think $x$ should be equal to $y$ (this is clearly incorrect). Also, the mass of reactants used is not mentioned, so I don't know how to proceed with the question.

## 1 Answer

Step 1: Balance the equation. Your equation as written $\ce{ Sb2S3 + F2 -> SbF5 + SF6}$ is not balanced. You do not have the same numbers of sulfur, antimony, or fluorine atoms on each side of the equation. You cannot say anything about equivalents or stoichiometry with an unbalanced equation. Hint: Start with $\ce{Sb}$ and $\ce{S}$ and balance $\ce{F}$ last.

Step 2: Determine the relationship between $x$ and $y$. Since $x$ is antimony and $y$ is sulfur and they are in the same compound initially, they have a relationship (but they are not equal). In $\ce{Sb2S3}$, there are 2 sulfur atoms for every 3 antimony atoms. What does that tell you that the ratio of $\frac{x}{y}$ must be?

Step 3: Use what you now know about $x$ and $y$ and the correct stoichiometric equivalents (which should give you $z$) to show that $x+y=z$.

• I think you have confused equivalents with moles, there is no need to balance the reaction while applying equivalent concept. Please read this page askiitians.com/iit-jee-chemistry/physical-chemistry/… – Ayush Pateria Jan 7 '14 at 20:02
• I'm not sure I have. Equivalents in the definition you link to have to do with solution concentration. You can also use 'equivalents' to refer to ratios of reagents added to a reaction: "bromobutane (1.0 equiv.), Mg (1.2 equiv.), and acetone (4 equiv.) were added...". In this case the equivalents are dependent on the stoichiometric ratio (i.e. if the ratio of bromobutane to Mgwas 1:2, then 1.2 equiv. of Mg would be 2.4 moles of Mg for every mole of bromobutane). This use of equivalents is arbitrary (amounts chosen by the experimenter). Thus I assumed this question was about stoichiometry. – Ben Norris Jan 8 '14 at 0:11
• I agree with @AyushPateria, balancing isn't required. Further, you said, "there are 2 sulfur atoms for every 3 antimony atoms" in step 2. It's 3 sulfur atoms for 3 antimony atoms right? I think x/y would be 2/3, but I don't understand your step 3. Could you elaborate on that? – Avasyu Jan 8 '14 at 5:08
• The law of equivalence states that one equivalent of an element combine with one equivalent of the other. In a chemical reaction, equivalents and milli equivalents of reactants react in equal amount to give same number of equivalents or milli equivalents of products separately. Accordingly in reactions like aA + bB -> mM + nN the following holds good : meq of A = meq of B = meq of M = m.eq. of N. In a compound MxNy meq of MxNy= meq of M = meq of N – Ayush Pateria Jan 8 '14 at 10:47