Question

Question #93ac1

Answer 1

Here's what I got.

Explanation:

The most important thing to look out for when dealing with a stoichiometry problem is if the chemical equation given to you is balanced. As it turns out, that is not the case here, i.e. your equation is unbalanced.

`"Na"_ 2"S"_ 2"O"_ (3(aq)) + "AgBr"_ ((s)) -> "NaBr"_ ((aq)) + "Na"_ 3["Ag"("S"_ 2"O"_ 3)_ 2] ""_ ((aq))`

In order to balance this equation out, you need `4` atoms of sodium, `"Na"`, on the reactants' side. You also need `2` thiosulfate ions, `"S"_2"O"_3^(2-)`, on the reactants' side. You can thus add a `color(red)(2)` coefficient in front of sodium thiosulfate, `"Na"_2"S"_2"O"_3`

`color(red)(2)"Na"_ 2"S"_ 2"O"_ (3(aq)) + "AgBr"_ ((s)) -> "NaBr"_ ((aq)) + "Na"_ 3["Ag"("S"_ 2"O"_ 3)_ 2] ""_ ((aq))`

Now, the balanced chemical equation tells you that every mole of silver bromide, `"AgBr"`, that takes part in the reaction consumes `color(red)(2)` moles of sodium thiosulfate and produces `1` mole of sodium bromide, `"NaBr"`.

The problem gives you grams of silver bromide, so right from the start you know that you must convert them to moles by using the compound's molar mass

`42.7 color(red)(cancel(color(black)("g"))) * "1 mole AgBr"/(187.77color(red)(cancel(color(black)("g")))) = "0.2274 moles AgBr"`

This many moles of silver bromide will consume

`0.2274 color(red)(cancel(color(black)("moles AgBr"))) * (color(red)(2)color(white)(a)"moles Na"_2"S"_2"O"_3)/(1color(red)(cancel(color(black)("mole AgBr")))) = "0.4548 moles Na"_2"S"_2"O"_3`

and produce

`0.2274color(red)(cancel(color(black)("moles AgBr"))) * "1 mole NaBr"/(1color(red)(cancel(color(black)("mole AgBr")))) = "0.2274 moles NaBr"`

Use the molar mass of sodium bromide to convert the number of moles to grams

`0.2274 color(red)(cancel(color(black)("moles NaBr"))) * "102.9 g"/(1color(red)(cancel(color(black)("mole NaBr")))) = color(green)(|bar(ul(color(white)(a/a)color(black)("23.4 g")color(white)(a/a)|)))`

You can thus say that the reaction of `"42.7 g"` of silver bromide produced `"23.4 g"` of aqueous sodium bromide and consumed

`"no. of moles of Na"_2"S"_2"O"_3 = color(green)(|bar(ul(color(white)(a/a)color(black)("0.455 moles")color(white)(a/a)|)))`

of sodium thiosulfate. Both answers are rounded to three sig figs, the number of sig figs you have for the mass of silver bromide.