Hey guys I did a lab and I have to answer the questions relating to the lab. I did question can someone please check if these are accurate

1. Calculation to Determine the molecular weight of unknown substance

Mass of unknown used: 2.0 g
Mass of water used: 50.0 g (0.05kg)
Kf = 1.86°C kg/mole
Experimentally determined freezing point of unknown: -1°C

Solution = delta Tf/-Kf
= -1°C/-1.86°C kg/mole
= 0.538 m (molality)

Molar mass = 2g / 0.05 kg * 0.538 m
= 74.4 g/mol

2. Calculations to determine i for KCl solution:

FOR PART 2 I AM NOT SURE WHAT IS THE RIGHT METHOD OF DOING IT, I DID IT TWO WAYS CAN SOMEONE PLEASE CHECK, WHICH IS THE RIGHT ONE.

Kf = 1.86°C kg/mole
Experimentally determined freezing point of KCl: -5.00
m KCl (given): 0.1 m (molality)

1. . Solution: i
= -5.00°C /(1.86°C kg/mole)(0.1)
i = 26.88
or

2. -5.00°C /-1.86°C kg/mole * 0.1m KCl
i = 0.2688

So is i 0.2688 or would it be 26.88

1. 👍 0
2. 👎 0
3. 👁 159
1. The first part looks ok to me.
The second part (i for KCl): You haven't shown any difference between the two methods.
delta T = i*Kf*m
i = delta T/K*m
i = 5.00/1.86*0.1 = 26.9 so your math for your first calculation is ok (but of course the correrct answer is nowhere near 26.9). The second calculation is EXACTLY the same set up so I don't know how you arrived at 0.2688.

1. 👍 0
2. 👎 0
2. 1. deltaTf = 0 - (-1.0) = 1.0 deg
Kf = 1.86
m = 1.0deg/1.86deg/molal unit
Final answer should be OK

2. i = (molality calculated from your experiemental deltaTf) / (molality calculated from moles KCl and kilograms of water)
Your answer (value of i) should be between 1 and 2, probably closer to 2

1. 👍 0
2. 👎 0
posted by GK
1 mole of KCl produces 2 moles of particles (ions). Because of this, a 1.00m solution of KCl would behave like a 2.00m solution as far as freezinf point, boiling point, and other colligative properties are concerned. That assumes 100% dissociation of KCl ionto ions in solution. The FPD would be (2.0)(1.86)=3.72) and the freezing point would be 0-3.72 = -3.72 deg.C. The actual freezing point lowering is less than predicted (assuming 100% dissociation) because some "ion pairs" behave like a single particle. Your 0.1m solution of KCl should produce the FPD of a 0.2m solution (-0.2*1.86=-0.376). Your definition, i = deltaT/Kf.m, is ok but it is producing an answer that is not reasonable. Probably poor experimental data.
(molality in terms of moles of ions) divided by (molality in terms of formula units)should give something less than (2 ions)/(formula unit).

1. 👍 0
2. 👎 0
posted by GK
4. thank you

1. 👍 0
2. 👎 0
posted by Saira

Similar Questions

1. Chemistry

Hey guys I did a lab and I have to answer the questions relating to the lab. I did question can someone please check if these are accurate 1. Calculation to Determine the molecular weight of unknown substance Mass of unknown used:

asked by Saira on February 3, 2010
2. chemistry, freezing point depression

Hey guys I did a lab and I have to answer the questions relating to the lab. I did question 1 & 2, can someone check if they are correct, and question 3 I don't know how to do it. 1. Calculation to Determine the molecular weight

asked by Sev on February 6, 2009
3. chemistry

Hey, I was just struggling with a question that I need to figure out to help me with my next lab, so if any of you guys could help me that would be greatly appreciated. Question: write a balanced equation for the reaction between

asked by alex on March 4, 2013
4. History (Unit 2 History exam Unit 2 Lesson 10)

Hey guys! I need help in my History unit 2 exam for 8th grade, I was wondering if any of you took it yet and could help me with some of the questions... I really need help it's very hard! >.< Can u pretty please help? T-T

asked by Kiki on November 4, 2015
5. chemistry

hey guys im stumped and burning out. Its mid term time. I got 5 midterms and lab test. i think im making this more complex than needed what is 10mg/dL Ca in mEq/L can u show me a simple way using the valence ? My answer doesnt

asked by Jim_R on February 7, 2010
6. 7th Grade Science

Hi, I really need help with this. I'm supposed to find information about biodiversity and fishing or housing and answer these two questions: "What challenges does [your topic] pose to biodiversity? What do you think should be done

asked by Music Lover 14 on March 18, 2016
7. Chemistry------DrBobb..

Lead metal is added to 0.140 Cr^3+(aq). Pb(s)+ 2Cr^3(aq) Pb^2+(aq)+2Cr^2+(aq) KC=3.2*10^{-10} A. What is Pb^2+ when equilibrium is established in the reaction? B. What is Cr^2+ when equilibrium is established in the reaction? C.

asked by Saira on February 7, 2009
8. physics

hey you guys , i reay appreciate it if you guys help me out . Q: a vector is a physica quantity that has which of the following properties ( can be more than one) a)magnitude b)color c)speed d)direction

asked by kevin on April 12, 2012
9. biology

In a pre-lab for nutrient food testing , I was assigned to answer these questions, but I have no idea what they are talking about. And no, we have not discusses the answers in class. Here are the questions: Answer the following

asked by Kayla on September 26, 2017
10. English

Hey guys, I was just wondering if you could help me find or explain to me media influence in the Hunger Games. I know this is a big part of the story, but I really need clarification. Thanks a lot guys!

asked by Alex on January 13, 2011

More Similar Questions