# Stoichiometry calculations

1.nitric acid reacts with potassium carbonate according to the following reaction
2(HNO3) + k2CO3 ----> 2(KNO3) + H2O + CO2
if nitric acid is reacted with 50.0 g of potassium carbonate, find the mass of water produced

2. manganese is added to steel to increase strength and resistance to wear. A 2.35 g sample of manganese steel is examined for composition and yields manganese in the form of manganese sulphate. if there is 0.322 g of manganese sulphate, calculate the percentage of manganese in the steel sample

3. a compound was found to contain 62.5% Pb, 8.5% N and 29.0% O. calculate its empirical formula.

4. If epsom salt MgSO4.zH2O is heated to 250 degrees celsius all of the water of hydration is lost. After heating a 2.04 g sample of the hydrate, 0.989 g of anhydrous MgSO4 remains. How many molecules of water are there per formula unit of MgSO4?

For #2, you simply need to figure out how much of that 2.35g sample is actually Mn, and then use that in the percent yield formula over the total amount.

The idea on #3 is to treat the percents like grams, and work out how many moles those "grams" would produce. Use those numbers (you have to sort of round them to the nearest whole usually) in your equation to find the empiracle formula and since you are not given any other variables it is impossible to find the molecular formula.

Number 4 is yet another percent yield question, this time utilizing finding mass by difference.

How are you approaching number one? If you show me what you have so far I can perhaps tell you where you are going wrong.

i have completed question 3 and 4, but i am still having difficulties with question 1 and 2

for question 2 i was thinking about finding the mol of manganese in manganese sulphate then finding the mass but im not sure what do do after that

for question 1 i have no idea where to start i was going to find the mol of potassium carbonate and go from there but im not sure where to go haha

Borek
Mentor

for question 2 i was thinking about finding the mol of manganese in manganese sulphate then finding the mass but im not sure what do do after that

If you have mass of manganese finding mass percentage in the original sample is just a plug and chug.

for question 1 i have no idea where to start i was going to find the mol of potassium carbonate and go from there but im not sure where to go haha

Do you know how to read reaction equation?

for question one there was one more bit of information but my teacher told me to cross it out and not use it

i had an attempt at question one using the other bit of information; if 6.00 x 10^-1 mol of nitric acid is reacted with 50.0 g of potassium carbonate

i worked out that the limiting reagent was HNO3,
number of mol of (H2O) = 6.00 x 10^-1 mol
i then worked out the mass of H2O; ((6.00 x 10^-1)/2) x the molecular mass of H2O which is 18.016

and i got a final answer of 5.4048 g of H2O is produced

please tell me if i did something wrong and if i should have used this other bit of information
thanks

Borek
Mentor
If you are given amount of nitric acid this becomes a limiting reagent question. What you did is correct, but if you were asked to ignore this information, calculate the result just assuming nitric acid was in excess.

fascinating, how you know how to do more difficult, limiting reagents question, but you have no idea how to do the simple stoichiometry. That usually means there are holes in the knowledge.

the reason my teacher left out the other bit of information was because he hasnt taught us about stoiciometry only about the mole, and this was a form of revision for the mole
i just taught myself how to do the limiting reagent stuff so i could use that other piece of information because i wasnt sure how to go about the question with out the other piece of information