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Chemistry problem, finding the mass percentage.In an experiment a piece of magnesium...

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hannahbannna | Student, Grade 10 | (Level 3) eNoter

Posted January 4, 2012 at 6:41 AM via web

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Chemistry problem, finding the mass percentage.

In an experiment a piece of magnesium ribbon is burned in air in a crucible. The mass of the mixture of MgO and magnesium nitride 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 was the mass percentage of magnesium nitride in the mixture obtained after the initial burning? Thank you!

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shik6991 | Student, Grade 10 | (Level 1) eNoter

Posted January 5, 2012 at 10:31 PM (Answer #1)

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najm1947 | Elementary School Teacher | (Level 1) Valedictorian

Posted January 28, 2012 at 12:22 PM (Answer #2)

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Magnesium Ribbon burning & conversion to MgO as Final Product

The products of burning of Magnesium Ribbon in air are Magnesium Oxide MgO and Magnesium Nitride Mg3N2.

In the subsiquent reaction of Nagnesium Nitride Mg3N2 with water, it converts to MgO. We see that 1 molecule of Mg3N2 converts to 3 molecules of MgO.

molar weight of MgO = 40.31 g/mole

molar weight of Mg3N2 = 100.95 g/mole

Weigth of MgO and Mg3N2 after burning = 0.470 g

Weight of the final product MgO = 0.486 g

Let x be the number of moles of MgO in the burning reaction

and y be the number of moles of Mg3N2 in the burning reaction

Therefore after reaction with water, the number of moles of MgO converted from Mg3N2 will be 3y.


40.31x + 100.95y = 0.470 .......(1)

40.31x+ 40.31(3y) = 0.486  .......(2)

solving above equation by subtrcting (1) from (2),

we get: 19.98y = 0.016 or y=0.000801

Weight of Magnesium Nitride in the mixture after burning = 100.95y

= 100.95*0.000801 = 0.0808 g

Mass percentage of Magnesium Nitride in Mixture

= 0.0808/0.470*100

= 17.2%

Note: molar weights of Magnesium Oxide MgO and Magnesium Nitride Mg3N2 are available on internet

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