I interpreting my results for a thermal decomposition lab

[littlebearrrr]

Homework Statement

I performed an experiment recently, which required me to heat a flask and an unknown compound to constant weight. The unknown was supposed to be a mixture of calcium sulfate dihydrate and NaCl. My goal for this lab was to determine the % NaCl in the mixture. My percent error wasn't too bad: I got 12%. I'm just curious as to what could have contributed to this, especially since this error was consistent among the rest of my lab group. (Note: I was sure to completely transfer all of my compound to my flask). Does it have anything to do with the hydrate not being completely driven off?

Homework Equations

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The Attempt at a Solution

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[Borek]

unknown was supposed to be a mixture of calcium sulfate dihydrate

Composition of hydrates is often only approximate and depends on the temperature, humidity and sample history.