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# Mass for enthalpy change of solution watch

1. I'm so confused as to what mass I use when calculating the energy change that occurs when a solid is added to water.....
Like if you added eg 5g na2co3 to 50cm3 water (and want to calculate the enthalpy change of solution) then I always thought you just used 50g as the mass? But then I did an experiment recently and apparently the mass uses should be 55g? And this is true whether the 5g of na2co3 is hydrated or anhydrous?
Like this just goes against anything I thought was correct before. So what should I be doing...... And is there a situation in which using only the mass of water is correct?

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2. (Original post by maths_4_life)
I'm so confused as to what mass I use when calculating the energy change that occurs when a solid is added to water.....
Like if you added eg 5g na2co3 to 50cm3 water (and want to calculate the enthalpy change of solution) then I always thought you just used 50g as the mass? But then I did an experiment recently and apparently the mass uses should be 55g? And this is true whether the 5g of na2co3 is hydrated or anhydrous?
Like this just goes against anything I thought was correct before. So what should I be doing...... And is there a situation in which using only the mass of water is correct?

Posted from TSR Mobile
Yes when the solute(Na2CO3 in this case) is HYDRATED so when the solute is dissolved in water, the mass of water from the solute adds to the overall mass of water.

Anhydrous you use JUST THE MASS OF WATER

also pls don't quote me on these, i'm pretty sure this is right but there's an uncertainty of about 10%
3. I'd check past paper mark schemes for your exam board. There are sure to be questions very similar to your problem. The MS will say whether you should include the solid, should not include it, or allow either answer. Just do what the MS recommends.

If the Q states that the specific heat capacity of an aqueous solution is 4.18 (like OCR A does), you should include it as the solid becomes part of (and hence increases the mass of) the aqueous solution.
4. Thank you both

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