When are drops of indicator most likely added to the erlenmeyer flask?
a. when preparing the burette
b. when filling the pipette with acid
c. when recording the initial burette volume
d. when preparing to measure ph change

The answer is: D.when preparing to measure pH change.

Near equivalence point indicator should change color, so we must pick indicator who change color near pH of equivalence point.

Equivalence point is the point which there is stoichiometrically equivalent amounts of acid and base.  

For example, phenolphthalein is colorless in acidic solutions and pink in basic solutions. If this indicator change color to dark red, more base is added and endpoint is not accurate.

Chemist can draw pH curve (graph showing the change in pH of a solution, which is being titrated) for titration and determine equivalence point.

answer: semi-infinite solid initially at a uniform temperature of 400 ◦c is cooled at its surface by convection. the cooling fluid at a constant temperature of 200 ◦c has a convective coefficient (ha) of 250 w/m2.k. the physical properties of the solid: k = 20 w/m.k and α = 4 * 10-5  m2/s. using δx = 0.04 m and δt = 10 s. write the finite difference equations describing the transient heat transfer of the first five δx and the temperature profile after 30 s.