 by Eidur K [on & off]

Question by Uqba A: How many grams of orange crystals are needed to produce 4.58g of water?
Ammonium dichromate (NH4)2CR2O7 which exist as orange crystals, may be decomposed upon heating according to the following reaction:
(NH4)2Cr2O7 -> Cr2O3 + N2 + 4H2O

How many grams of orange crystals are needed to produce 4.58g of water?

what i did is this:

252 x 4.58 / 72
= 16.03

which gives the same answer, is that the right way ?

exactly 6.3
I learned this along time ago so I didn’t need ur reaction formula anyway.

#### One Response to How many grams of orange crystals are needed to produce 4.58g of water?

• Dennis M says:

(NH4)2Cr2O7 -> Cr2O3 + N2 + 4H2O

First find the molecular mass of (NH4)2Cr2O7 = 2(14) + 8(1) + 2(52) + 7(16) = 252 grams / mole

Next find how many moles of H2O are produced = 4.58 grams / 18 grams mole H2O = 0.254 moles H2O

Notice from the balanced equation that one mole of (NH4)2Cr2O7 produces 4 moles of water. Therefore the moles of (NH4)2Cr2O7 is only one fourth the number of moles of water produced = 0.254 moles / 4 = 0.0636 moles (NH4)2Cr2O7

Now find the mass of (NH4)2Cr2O7 needed:

Mass (NH4)2Cr2O7 = 0.0636 moles X 252 grams / mole = 16.0 grams

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