Michael V. answered • 12/14/21
Chemistry Graduate with Educational and Industry Experience
When you approach questions like these you need to think about what you know and what you don't know.
You Know
You have 25.0 grams of NH4CL
You Don't Know
How many grams of NH3
How many grams of HCl
You Can Figure Out
Whether or not the equation is balanced
The Molar Mass of NH4Cl
The Molar Mass of NH3
The Molar Mass of HCL
You will need to determine the Molar Masses of the 3 chemicals. Use a periodic table.
EX: H2O, H has a mass of about 1 (There will be 2), O has a mass of about 16. The total mass would be M = (1x2) + (16x1) = about 18 grams/mole
You may need to use more significant figures in your answer but this is a good ballpark.
Next you will need to determine the number of moles of NH4Cl you have. Divide the grams you have by the molar mass in order to cancel out the grams and leave moles.
EX: If you have 20 grams of H2O you would have 20 grams / 18 grams/mole = 1.11 moles of water
Then you can look at the equation and determine how many moles of each reagent you will need.
EX: 2H + O -> H2O
This is a balanced equation so I don't have to change anything
You will need 2 moles of H for every mole of H2O, 1.11 moles x 2 = 2.22 moles of Hydrogen
You will need 1 mole of Oxygen for every mole of H2O, 1.11 moles x 1 = 1.11 moles of Oxygen
The last thing you need to do is convert the moles to grams by using their molar mass
EX: 2.22 moles of Hydrogen, 2.22 moles x (1 gram / mole) = 2.22 grams of Hydrogen
EX: 1.11 moles of Oxygen, 1.11 moles x (16 grams / mole) = 17.76 grams of Oxygen
This sounds about right and when I add the two masses together I get about 19.98 grams
This is a little less than the 20 grams I should get (because mass can not be destroyed) but it can be explained by the number of significant figures I didn't use.
Michael V.
Here it really depends on the question. Once you have determined the number of moles of each compound you could just stop but it is asking for grams (which is more practical to work with). But a mole of Ammonia doesn't have the same mass as a mole of Hydrochloric Acid or Ammonium Chloride. In my example with making water it should make sense that even though you need twice as many moles of Hydrogen, the Oxygen is so much heavier that half as many atoms weighs way more. One reason we use moles is because it lets us use ratios easier but you can't just open up a bottle and add 3 moles of something unless you weighed it earlier.12/15/21
Mert U.
Why do I need to calculate all molar masses not only NH4Cl?12/14/21