Question

Find mass of excess reactant. 3Cu+8HNO3-->3CU(NO3)2+2NO+4H2O. I found the mass of H2O which would be 1.5g. But I am not getting the right mass of the excess

Answer 1

PLEASE SOMEONE HELP GOD WILL BLESS YOU

Answer 2

@nincompoop

Answer 3

11g of Cu and HNO3 were used !

Answer 4

ugh :(

Answer 5

okai so you have your equation
Find which is limiting and which is excess

Answer 6

like in mol

Answer 7

Limiting is Cu with 0.173 moles. and Excess is HNO3 which is 0.174 moles

Answer 8

(limiting would be the smaller number right?)

Answer 9

yes it's the smaller number
lemme check that

Answer 10

alright

Answer 11

they don't react in 1:1 ratio--it's 3 Cu + 8 HNO3, so 3:8 ratio.

Answer 12

so what would I do ?

Answer 13

for HNO3, use 0,175mol instead

Answer 14

it was 0,1745
so yeah
anyways now that you found your limiting and excess....

Answer 15

calculate how much excess is consumed in the reaction
Excess is HNO3
Use mole ratio to determine it
The equation follows:
mol of limiting*(mole ratio) = mol of excess used up

Answer 16

so what is the problem?

Answer 17

so 0.173moles of limiting *(3/8) which gave me 0.064 :S

Answer 18

could you show me how you are solving this?

Answer 19

incorrect
it's 8/3
and ill show you a sample nincompoop

Answer 20

...after

Answer 21

so I got 0.461

Answer 22

what sample? just show me the dimensional analysis

Answer 23

alright lemme draw it out
in the meantime
use that mole value you got
and subtract it from your original excess

Answer 24

wait, nope, not gonna work...okaay so ill just draw it out

Answer 25

okay so 0.4613-0.175 = 0.2863

Answer 26

um okayy

Answer 27

what difficulty are you having converting moles to grams?

Answer 28

well I'm not getting the right answer soo....

Answer 29

I dont know what im doing wrong

Answer 30

make your life easier, write down the masses of each reactants
how many grams are there in one mole of Cu?

Answer 31

1. List Ratio
2. List Molar Masses
3. Find Limiting and Excess
4.Find how much mole of excess is consumed
5. Subtract from original moles
6. Convert value to desired unit

Answer 32

start using dimensional analysis

Answer 33

i never really learned it..
so i made a list

Answer 34

ohkaaaaaaay nin

Answer 35

Okay so this is what I have so far guys. 11g of Cu so 0.173 moles. (limiting) and 11g of HNO3 so 0.174 moles. 0.174*(4H2O/8HNO3) = 0.087 x 18.0.148 which gave me 1.5g mass for H2O

Answer 36

but you want mass of excess reactant

Answer 37

Then I said I have 0.174moles - 0.087moles needed = 0.087 moles x 63.0119 (HNO3) which gave me 5.5g but thats wrong.

Answer 38

what am i doing wrong

Answer 39

hey let me show you something that helps
how do you turn a rabbit into frog? \[Rabbit \times \left( \frac{ Frog }{ Rabbit } \right)= Frog\]

Answer 40

uh cancel out the rabbits o.o

Answer 41

Wait can you do the question like type it out and show me step by step what you did?

Answer 42

that's what dimensional analysis is. can you show me your solution using dimensional analysis so we can pick up where you are having a problem.

Answer 43

I will attach a picture and show you what I did because I didn't really learn that way

Answer 44

you have to learn that way so when you go back you can always trace where the error is.

Answer 45

can you see?

Answer 46

it should look like this \[3molCu \times \left( \frac{ 63.546 gCu }{ 1molCu } \right) = \]

Answer 47

you cancel out the mol and you multiply 3 by the gram value of one mole of Cu.

Answer 48

so 190.638?

Answer 49

yes, 3 moles of Cu = 190.638
you're just converting values from moles to grams, don't get excited that's not the answer yet

Answer 50

lool okay so whats the next step?

Answer 51

what do you think is the next step?

Answer 52

you're identifying the excess right?

Answer 53

yeahh

Answer 54

okay how would you achieve that? define excess

Answer 55

excess, meaning the leftover? the extra? Soo would I subtract from something?

Answer 56

the limiting or mass of h2o ?

Answer 57

\[8molHNO_3\times \left( \frac{ ?gHNO_3 }{ 1molHNO_3 } \right) = \]

Answer 58

I got 504.0952

Answer 59

if I were to make a bicycle. how many wheels do I need for every frame?

Answer 60

2?

Answer 61

if I were to make bicycles and I have 30 wheels and have 10 frames. how many bicycles I can make?

Answer 62

I'm guessing 15? Or wait how many frames does one bike have? 2? is 2 then 20 bikes..

Answer 63

NO omg I feel so stupid now

Answer 64

no… for every frame you need 2 wheels to make 1 bike
10 frames = 20 wheels
this gives you 10 wheels extra

Answer 65

oh..okay

Answer 66

you get this part right?
how about if I had 10 wheels and 10 frames? how many bikes I can make?

Answer 67

5? because you wouldn't have enough wheels?

Answer 68

awesome!
in the first bicycle question, your limiting would be frames; in the second bicycle it would be the wheels.

so basically your limiting is always the thing that runs out first and of course the other would be your excess.

Answer 69

is this making any sense and are you able to draw connection from bikes to chemistry?

Answer 70

yess

Answer 71

do you think you can take a stab at the question once more, but this time using the dimensional analysis?

Answer 72

Um. Well you said frames = 2 wheels. and frames is my limiting. So my limiting is 190.638 and my excess is 504.095. Would I divide?

Answer 73

if you're looking for the ratio then you divide

Answer 74

Soooo 504.095/190.638 ?

Answer 75

WAIT I GOT 2.64

Answer 76

omfgg thats it?

Answer 77

is taht it?

Answer 78

it took my 2 hours to get to that? and all I had to do was divide the things ? wooooww. thanks tho !

Answer 79

you work your way back. this is a good example
http://www.chem.tamu.edu/class/majors/tutorialnotefiles/limiting.htm

Answer 80

you're way ahead… lol I told you that you're just merely converting

Answer 81

man but I tried this with another question and it didn't work. I'm getting so fusterated now

Answer 82

what did I do wrong again? =l

Answer 83

lol you didn't even finish the first problem yet

Answer 84

or did you?

Answer 85

wait I didnt? oh no :(

Answer 86

did you read the link I sent you? it's a good example, but I can show you how it is done from textbooks.

Answer 87

after I divied 504.0942g/190.638g I got 2.64 g

Answer 88

isn't that the answer?

Answer 89

It's 2am, I just wanna know how to do it straight forward :/ I havea test tomorrow and I'm very nervous about it

Answer 90

check page 107 of that book

Answer 91

alrighty thank you! :)

Answer 92

wow this book is good, which country are you from? damn I wish we had these

Answer 93

oh so theorictical is the limiting? and then they give you actual?

Answer 94

you're welcome. There's a reason why I made sure you understood the bike problem first, because it is the very same concept you will use for stoichiometry
4 wheels + 3 frames ----> 2 bikes + 1 piece of metal

you identify how many wheels would it require you to make 2 bikes
you do the same with frames
you identify the limiting reagent or excess
you convert their values to grams (or to moles if you were given in grams initially)

Answer 95

no. theoretical yield versus actual yield. theoretical is what would you produce assuming 100% efficiency, actual well of course if you did the actual experiment, how much would you produce while factoring the spills, mistakes etc.