Chemistry 120 Lab Question

Question

Would your experimental n be higher or lower than the true value if you made the following errors in this laboratory?

a. You are in a hurry to leave the laboratory and you do not achieve a constant mass with heating.

b. The crucible is tilted too much during heating and some of the sample falls out.

c. The lab instructor accidently contaminates your hydrate sample with NaCl before giving the sample to you.

Answer 1

Short answers: (a) lower, (b) higher, (c) lower. The reasoning matters more than the answers, so here is the method that settles any version of this question.

You found n from two weighings. Writing it out is what makes each error obvious:

n = mol H₂O / mol anhydrous salt = [(m_hydrate - m_residue) / 18.02] ÷ [m_residue / M_anhydrous]

Only two measured numbers appear: the mass before heating and the mass after. So for any error, ask one question. Which of those two masses does it corrupt, and in which direction? Then read the effect off the formula.

(a) You did not heat to constant mass. Some water is still trapped in the sample, so the residue weighs more than true anhydrous salt. That makes m_residue too high, which does two things at once: the measured water loss (m_hydrate - m_residue) comes out too small, shrinking the numerator, and the calculated moles of salt come out too large, inflating the denominator. Both push the same way, so n is too low. This is why the procedure makes you heat, cool, and reweigh until two masses agree; constant mass is the only evidence that all the water actually left.

(b) Sample fell out of the tilted crucible during heating. Your before mass was recorded correctly, but the after mass is now missing material that never evaporated. m_residue is too low, so the apparent water loss is too large and the numerator inflates, while the moles of salt come out too small and the denominator shrinks. Again both effects reinforce, so n is too high. The calculation cannot tell the difference between mass that left as steam and mass that left as spilled solid, and it credits all of it to water.

(c) The sample was contaminated with NaCl before you got it. This one is the most interesting, because your water measurement is actually fine. NaCl is not a hydrate and loses nothing on heating, so it sits there through the whole procedure and cancels out of the difference m_hydrate - m_residue. The numerator is correct. But the residue you weigh is real anhydrous salt plus the NaCl, so dividing that whole mass by M_anhydrous makes you think you had more salt than you did. Denominator too large with a correct numerator gives n too low.

Worth noticing that (a) and (c) both come out low but for genuinely different reasons: in (a) you underestimated the water, in (c) you overestimated the salt. Getting the direction right by luck is not the same as understanding the error, and lab exams tend to ask exactly that distinction.

A general habit for every error-analysis question of this type: never reason about the answer directly. Push the error into whichever measured mass it touches, then let the formula tell you which way n moves. Intuition about final answers is unreliable; the algebra is not.