[Cyberconvict]

I'm not sure why I can't figure out this problem and would like some help with the set-up of the equation. The problem goes as follows: In an experiment a piece of magnesium ribbon is burned in air in a crucible. The mass of the mixture of MgO and Mg3N2 after burning is 0.470 g. Water is added to the crucible, further reaction occurs, and the crucible is heated to dryness until the final product is 0.486 g of MgO. What is the mass percentage of magnesium nitride after the initial burning?

I know the balanced equation is Mg3N2(s) + 3H2O(l) --> 2NH3(g) + 3MgO(s), but am unsure as to where to go from there. I tried to take the difference between the two and perform a limiting reagent like calculation, but that gave me the wrong answer. The back of the book says 17%.

If possible could you show me the process with this one and give me another example problem similar to this?

[Borek]

Back of the book answer is OK.

Your reaction is OK.

Sample mass increased by 0.016g

If there is 1 mole of Mg₃N₂ - what is mass increase?

How many moles of Mg₃N₂ was in the original sample?

What was its mass?

Note you can omit mole calculations and do all calculation based on molar masses.

[Albert]

My hint is: use a system of two equations. I've always liked algebra

[Cyberconvict]

Back of the book answer is OK.

Your reaction is OK.

Sample mass increased by 0.016g

If there is 1 mole of Mg₃N₂ - what is mass increase?

How many moles of Mg₃N₂ was in the original sample?

What was its mass?

Note you can omit mole calculations and do all calculation based on molar masses.

There's a 20.013g increase in weight per every mole of Mg₃N₂ that reacts with water. A 0.016g increase occured in this problem, thus 0.016/20.013=7.995E-4 mol of Mg₃N₂. That means .081g of Mg₃N₂ was in the sample and .081g/.470g=.17=17% Mg₃N₂ in the original sample.

Alright I figured this out, but could you give me another sample problem, because I can't find any similar ones in my book.

[Borek]

These two are different, but close in the way of thinking needed:

1.35g mixture of CaO and CaCO₃ is dissolved in hydrochloric acid, then sulfuric acid is added. Dried precipitate weights 2.32. What was the composition of the sample?

1.000g of iron filings is added to copper sufate solution. Once the copper is replaced from the solution fillings are dried and weighted - they weight 1.030g. What mass of iron was dissolved in the solution during reaction?