Hi, I was watching this video: https://www.youtube.com/watch?v=BSYPYLdsbBg. In it, the teacher said the half-filled ECs, molybdenum, and chromium would be different, but she said tungsten's EC wouldn't. Why does this only apply to molybdenum and chromium but not tungsten? I'm a little confused.
- Aditi

its working now, thank you! 

You can express  it as heat lost by Al or heat gained by water.
I think they've given answer in terms of heat gained by water.

Hi Dr Uma,

I came across a free-response question from the Princeton Review. However, I could not get (c) because they used the value 380 J instead of -380 J. Wouldn't that contradict (b), as to calculate the specific heat of Al we used -380 J. Also, it is the cooling of aluminum so shouldn't it indicate an exothermic process?   

ohh! thank you, i forgot , i will go back and revise that unit.

Let us check
81g HF * 1mol /20g HF * 540kJ /2 mol HF
Yes you are right it is 1100 kJ

In this question we need to work out the energy of fuel for 153 g  from the information of 100% C mass  and 100% H mass.
Charcoal – 100% carbon so 34kJ/g*153g
Hydrogen – 100% H so 142kJ/g * 153g
Wood pine – 50% C and 6% H = ( (50*153)/100*34kJ/g + (6*153 )/100*142) kJ
Bituminous coal  – 7% C and5% H = ( (7*153)/100*34kJ/g + (5*153 )/100*142) kJ

Crude oil  – 85% C and 12 % H = ( (85*153)/100*34kJ/g + (12*153 )/100*142) kJ

Natural gas  –70% C and 23 % H = ( (70*153)/100*34kJ/g + (23*153 )/100*142) kJ

Now Check which one is giving maximum energy

Aditi Molarity is a way of expressing how much solute is present in the given solution. Basically is is the way we express concentration of solute in solution.
By definition
Molarity = moles/ volume in L = mmol /volume in mmol
If you remember we did questions based on it in unit 4.

Hi Dr. Uma,

I was wondering on how I should solve this problem?

Hi Dr. Uma,

I was wondering about the question linked to this post. Somehow I keep getting around 1100 kJ, however, the answer says it is 2200 kJ.