Where Did the 400g in Enthalpy Change of Neutralization Come From?

[Iwanttolearnphysics]

Homework Statement

Calculate the enthalpy change of neutralization for the following reaction

Relevant Equations

q = mcΔT

Hi, everyone! There's a question I found on website I'm using and the answer key here is given. My question is this, where did the 400g come from?

According to the definition of enthalpy of neutralization (chem libretexts), the standard enthalpy change of neutralization is the enthalpy change when solutions of an acid and an alkali react together under standard conditions to produce 1 mole of water.

I highlighted 1 mole of water because that's what I used to solve the problem.

This is what I did step by step:

• First, I wrote the balanced chemical equation. I know that 0.800 mol of NaOH and 0.800 mol of HCl was consumed. From the balanced chemical equation, I concluded that 0.800 mol of water must have been produced too.
• 0.800 mol of water is equivalent to 14.4g of water.
• I used q = mcΔT and I wrote:
  • q = 14.4g x 4.18 x (27.7-25.1)
  • q = 156.50 J of heat was released per 0.800 mol
  • If I want to get per mol, then 156.50J/0.800 mol = 195.62 J/mol

What did I do wrong? Thank you.

 

[Borek]

What was heated up: water produced, or whole solution?

 

[Iwanttolearnphysics]

Sorry for the late reply. I was trying to find ways to solve this problem on my own and I still don't get it. Yeah, it was the whole solution that was heated up. But I still don't get why it's 400g?

I used the molar masses to calculate the masses in grams of the following compounds and these are what I got:

• 1 mol of NaOH = 32g
• 1 mol of HCl = 36g
• 1 mol of H2O = 18g
• 1 mol of NaCl = 58g

32 + 36 + 18 + 58 = 144g

Where did the 400g come from?

 

[Borek]

What is the volume of the solution?

What is the solution density (yes, we need some approximations here)?

What is the solution mass?