<span>6.50x10^3 calories.
Now we have 4 pieces of data and want a single result. The data is:
Mass: 100.0 g
Starting temperature: 25.0°C
Ending temperature: 31.5°C
Specific heat: 1.00 cal/(g*°C)
And we want a result with the unit "cal". Now you need to figure out what set of math operations will give you the desired result. Turns out this is quite simple. First, you need to remember that you can only add or subtract things that have the same units. You may multiply or divide data items with different units and the units can combine or cancel each other. So let's solve this:
Let's start with specific heat with the unit "cal/(g*°C)". The cal is what we want, but we'ld like to get rid of the "/(g*°C)" part. So let's multiply by the mass:
1.00 cal/(g*°C) * 100.0 g = 100.0 cal/°C
We now have a simpler unit of "cal/°C", so we're getting closer. Just need to cancel out the "/°C" part, which we can do with a multiplication. But we have 2 pieces of data using "°C". We can't multiply both of them, that would give us "cal*°C" which we don't want. But we need to use both pieces. And since we're interested in the temperature change, let's subtract them. So
31.5°C - 25.0°C = 6.5°C
So we have a 6.5°C change in temperature. Now let's multiply:
6.5°C * 100.0 cal/°C = 6500.0 cal
Since we only have 3 significant digits in our least precise piece of data, we need to round the result to 3 significant figures. 6500 only has 2 significant digits, and 6500. has 4. But we can use scientific notation to express the result as 6.50x10^3 which has the desired 3 digits of significance. So the result is 6.50x10^3 calories.
Just remember to pay attention to the units in the data you have. They will pretty much tell you exactly what to add, subtract, multiply, or divide.</span>
<span>Assume you have 1.00 L (1000 mL) of solution.
d = m / V
m = d x V = 1.23 g/mL x 1000 mL = 1230 g of solution
0.387 mol/L x 1 L = 0.387 mol HCl
0.387 mol HCl x (36.5 g / 1 mol) = 14.1 g HCl
mass of water = 1230 g solution - 14.1 g HCl = 1216 g H2O = 1.216 kg H2O
molality = mol HCl / kg water = 0.387 mol / 1.216 kg = 0.318 mol/kg (or 0.318 molal)</span>
Answer : The enthalpy change for the reaction is, 97.7 kJ
Explanation :
According to Hess’s law of constant heat summation, the heat absorbed or evolved in a given chemical equation is the same whether the process occurs in one step or several steps.
According to this law, the chemical equation can be treated as ordinary algebraic expression and can be added or subtracted to yield the required equation. That means the enthalpy change of the overall reaction is the sum of the enthalpy changes of the intermediate reactions.
The given main chemical reaction is,
The intermediate balanced chemical reaction will be,
(1) 
(2) 
(3) 
Now we are multiplying reaction 1 by 2 and reversing reaction 3 and then adding all the equations, we get :
(1) 
(2)
(3) 
The expression for enthalpy of main reaction will be,
Therefore, the enthalpy change for the reaction is, 97.7 kJ
