Posted by Jordyn on Wednesday, September 8, 2010 at 7:42pm.
Most stoichiometry problems are worked the same way. Here is an example of a problem I posted. Follow these steps.
http://www.jiskha.com/science/chemistry/stoichiometry.html
The link I gave you was not for a limiting reagent problem. This one is a limiting reagent problem and the procedure I gave you needs to be modified to make it work.
Write the equation and balance it.
Convert 17.24 g ZnO to moles. moles = grams/molar mass.
Convert 6.87 g HCl to moles the same way.
Convert moles ZnO to moles ZnCl2.
Convert moles HCl to moles ZnCl2.
It is likely that the two answers for moles ZnCl2 will not be the same. The correct answer in limiting reagent problems is ALWAYS the smaller value.
Use the smaller value of moles ZnCl2 and convert to grams ZnCl2. g = moles x molar mass.
Post your work if you get stuck.
17.24 g ZnO/81 g/mol= .2128395 mol
6.87 g HCl/36 g/mol= .190834 mol
?
You shouldn't round until the final step.
17.24/81.39 = ?? moles ZnO
6.87/26.46 = ?? moles HCl.
Now, using the coefficients in the balanced equation, convert moles ZnO to moles ZnCl2. (You can see how that is done by following the proper step in the first link I gave you.)
Do the same and convert moles HCl to moles ZnCl2.
Use the smaller number of moles and convert to g ZnCl2. g = moles x molar mass.
im lost.
Print this out. Remember the process.
This is a limiting reagent problem.
Step 1. Write the equation and balance it.
ZnO + 2HCl --> ZnCl2 + H2O
Step 2a. Convert 17.24 g ZnO to moles. moles = grams/molar mass
17.24/81.39 = 0.2118 moles ZnO
2b. Convert 6.87g HCl to moles.
6.87/36.45 = 0.1885
Step 3a. Using the coefficients in the balanced equat4ion, convert moles ZnO to moles ZnCl2.
moles ZnO x (1 mole ZnCl2/1 mole ZnO) = 0.2118 moles ZnO x (1/1) = 0.2118 moles ZnCl2.
3b. Same procedure, convert moles HCl to moles ZnCl2.
moles HCl x (1 mole ZnCl2/2 moles HCl) = 0.1885 moles HCl x (1/2) = 0.0942 moles ZnCl2
3c. moles from 3a and 3b are not the same which means one must be wrong. The correct one. in limiting reagent problems, is ALWAYS the smaller value which makes 0.0942 the correct value AND it means the HCl is the limiting reagent.
Step 4. Convert moles ZnCl2 to grams.
g = moles x molar mass
0.0942 x molar mass ZnCl2 = ??
I hope this helps.
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