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### Topic: Why can you use the Molarity to find the moles of an ion in a compound?  (Read 679 times)

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#### Kirby_B52

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##### Why can you use the Molarity to find the moles of an ion in a compound?
« on: February 05, 2020, 08:01:06 PM »
Hi! I have a question on this. In this problem, When 20.0mL of 1.00M NaCl at 22.00°C is mixed with 20.0mL of 1.00M AgNO3 at 22.00°C in a calorimeter, a white precipitate forms and the temperature of the mixture reaches 29.60°C. Assume that the specific heat capacity of the mixture is 4.184 J/g*C, its density is 1.00g/mL, and the volumes are additive.
a) Write the net ionic equation for this reaction.
b) Calculate ΔH for the reaction.
Why when calculating the ΔH, is the Molarity of AgNO3 used as how many moles of Ag+ there is when calculating from 0.0200L x 1.0mol Ag+/1L=0.0200mol Ag+. This is what is shown on the lecture about this and I don't understand why the molarity is used to find the moles to then use those moles to find the ΔH. Could you please explain? Thanks!

#### MNIO

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##### Re: Why can you use the Molarity to find the moles of an ion in a compound?
« Reply #1 on: February 06, 2020, 01:45:33 AM »
steps
(a) write the balanced equation
(b) convert volume and molarity for each reactant to moles of that reactant
(c) determine limiting reagent
(d) Calculate heat.... hint, Q = m * Cp * dT
(e) dH is often expressed as kJ / mol of reaction.  yes that's confusing
Let me see you try (a) - (d) and I'll help with (e)

#### Borek

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##### Re: Why can you use the Molarity to find the moles of an ion in a compound?
« Reply #2 on: February 06, 2020, 03:43:26 AM »
Two parts to your question and it is not clear what the problem is.

I don't understand why the molarity is used to find the moles

Molarity/moles conversion are just direct application of the molar concentration definition.

Quote
to then use those moles to find the ΔH

ΔH has to be reported in J per amount of substance - and amount of substance can be expressed using either its mass, or its number of moles. Thus you need to find either mass or moles of the substance that was produced in the reaction. Data given allows for the latter only, but that's OK - actually molar heats/enthalpies/whatever are used much more often.
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