Last few questions and I can finally get some sleep! 4 NO2 + 6 H2O 4. When a 28.7 gram sample of KI dissolves in 60.0 grams of water in a calorimeter, the temperature drops from 27.2°C to 13.2°C. Calculate the Detla H for the process. KI (s) — K+ (aq) + I- (aq)

Question

Last few questions and I can finally get some sleep! <3

1. How much heat energy is absorbed by 5.1 x 10^2 g of ice at -20°C as it is converted to water at 0°C?

2. What mass of water at 100°C can be converted to vapor at 100°C by the addition of 5.0 x 10^4 J of energy?

3. If 1.04 grams of NH3 are reacted, how many molecules of H2O are produced?

4NH3 + 7 O2 —> 4 NO2 + 6 H2O

4. When a 28.7 gram sample of KI dissolves in 60.0 grams of water in a calorimeter, the temperature drops from 27.2°C to 13.2°C. Calculate the Detla H for the process.

KI (s) —> K+ (aq) + I- (aq)

anonymous · Answer

@Abhisar & @Somy  Feel free to help if you can :p Most of this is going over my head as I missed some much class. Thanks! :)

somy · Answer

q 1
use this formula 
$$q = mc \Delta T$$

somy · Answer

c is constant which is equal to 4.18

somy · Answer

$$\Delta T$$
is change in temperature 
as for the q1 its changing from -20 to 0 so 
finial- initial = 0- (-20) = +20 
make sure your signs right because they show if the energy is released or absorbed

somy · Answer

im getting 42636 J

somy · Answer

same formula used in q 2 
but now u do know q value which is Joules  
but u do not know mass so make mass as subject

somy · Answer

im getting 119.6g for this question 2

somy · Answer

in q 3
first find mole of NH3 
u are given mass and Mr you can find from periodic table

eric_d · Answer

It shld be 0.06mol

somy · Answer

consider coefficient of NH3 which is 4

eric_d · Answer

so, 0.06/4=    mol

somy · Answer

$$1.04 \div (4\times17) = 0.015 moles of NH3 $$

somy · Answer

ration between NH3 and H2O is 4:6 
$$4 \rightarrow 0.015 moles$$
$$6 \rightarrow X moles $$
so mole of H2O will be 
$$(6 \times 0.015)  \div 4 = 0.0225 moles$$

somy · Answer

if your q in 3 is how many molecules of H2O are produced 
then multiply mole of H2O by Avogadro's number 
which is $$6.02\times 10^{23}$$

anonymous · Answer

Wow thanks.. Sorry I fell asleep. @Somy

somy · Answer

it's ok i figured as much :)

anonymous · Answer

so for question 3 I do what?

somy · Answer

find mole of NH3

anonymous · Answer

How do I do that?

somy · Answer

mole= mass/Mr

anonymous · Answer

whats mr? Do I need the periodic table for this?

somy · Answer

yes

somy · Answer

Mr is molecular mass

anonymous · Answer

ohhhh ok. so I need to add up the mr from the periodic table, what is the other mass from?

somy · Answer

it's given to u in the question

anonymous · Answer

Ok so 17.031g?

somy · Answer

?? u have grams given in the question

somy · Answer

look in the question the mass given for NH3 is 1.04g
u need to find Mr 
and then plug in the values into the formula :)

anonymous · Answer

17.031 g/mol is the molecular mass? right?

anonymous · Answer

So I got .061 moles NH3?

somy · Answer

Mr of N is 14 and of H is 1 
u have 1 N and 3 H 
so 14+3= 17 
u have coefficient 4 before NH3 in the reaction so 4*17= 68

somy · Answer

oh yeah 17.031 is also right, i just rounded of :)

somy · Answer

no it's gonna be 0.015moles

anonymous · Answer

o.O Lost me again. I suck at chemistry. How'd you get .015?

somy · Answer

look 
we said that coefficient of NH3 is 4
that means 4*17 = 68 -this is ur total Mr
now mole =mass/Mr
=1.04/68=

somy · Answer

4NH3 + 7 O2 —> 4 NO2 + 6 H2O
here in reaction, u see 4 before NH3 right?

anonymous · Answer

yeah, makes sense. So is .015 the final answer?

somy · Answer

yes 
thats for mole
now see the ratio between NH3 and H2O

somy · Answer

look at coefficients of each

anonymous · Answer

wait, so how many molecules of H2O are produced?

somy · Answer

u have to find moles of H2O first

anonymous · Answer

By the way - Sorry to be so quick and jumpy. I'm trying to finish this and get out the door so I'm not late.

somy · Answer

its ok
look ration between NH3 and H2O 
is 4:6 
(look at the equation u'll get it) 
so if 4 give 0.015moles how much will 6 give?

somy · Answer

here u do normal cross multiplication as in math

anonymous · Answer

.0225?

somy · Answer

thats right

somy · Answer

now they are asking number of Molecules 
so multiply this mole by Avogadro constant  6.02*10^23

anonymous · Answer

So: .0225 x 6.02 x 10^23? That game me 1.3545e22

somy · Answer

thats right

somy · Answer

1.3545*10^22

anonymous · Answer

so that's the final answer?

somy · Answer

yes

anonymous · Answer

YAY! Thanks so much @Somy :) Do you know the answer to number 4 by chance? I have to go so I can't solve it.

somy · Answer

ah no i didn't solve it yet

somy · Answer

q= mcT
q= 60* 4.18 * 27.2°C= 6821.76 J


q= 60* 4.18* 13.2°C.= 3310.56 J

notice temperature is decreasing so its exothermic reaction 
6821.76-3310.56= 3511.2 J 
but since its exothermic then it'll be -3511.2 J

somy · Answer

you are welcome btw :)

anonymous · Answer

So delta H = -3511.2 J

somy · Answer

yes

anonymous · Answer

YAAAAAY! Thank you <3 I love you soooo much right now. You just saved me :) I have to go but lets keep in touch...

somy · Answer

sure lol :D no problems :) 
are the answers in options though?

anonymous · Answer

No, I wish!

somy · Answer

oh that's sad, u didn't give the options so i did the questions the way i thought of them

somy · Answer

im sorry though T_T mb i made mistakes

anonymous · Answer

wait, made mistakes on what?

somy · Answer

no, i mean if none of my answers are matching your options

somy · Answer

mb i understood the questions wrong or made mistakes ( though i don't really see mistakes )

anonymous · Answer

@Somy  Oh no! I was saying these questions weren't multiple choice. There weren't any options. Just had to solve it.

somy · Answer

oh lol good then :) i thought i did all wrong for u T_T