"tear down this wall! " is a line from a speech made by u.s. president ronald reagan in west berlin on friday, june 12, 1987, calling for the leader of the soviet union, mikhail gorbachev, to open up the barrier which had divided west and east berlin since 1961.
first you should know that a dependent variable is the one which its values depends of the ones of other variable. in this case, the final mass is the dependent variable because this amount directly depends on how much acid the scientist pours in the initial mass to increase or decrease it. this is also why she remeasures it once the experiment is done.