Looking through my General Science solution guide it says the course should be weighted 35% lab grade and 65% test grade. Cumulative tests are worth twice as much as regular tests. "if you feel you really must include the study guides in the student's total grade, make the labs worth 35%, the tests worth 55%, and the guides worth 10%. A straight 90/80/70/60 scale should be used to calculate the student's letter grade.". It also goes on to say that the average college bound student will probably get a "C" grade.
While I don't own the chemistry text, I would assume the guidelines would probably be similar from the same publisher. I would feel weighting the grade this way would be appropriate as would weighting certain questions on the test higher than others provided that is determined before the test was taken and on rare occurances. Of course, you would only grade labs you actually did. My only concern is needing a lab science. Are there requirements or guidelines of a certain number of labs done? If so, I second the opinion of watching some video clips of experiments to make up the difference if you fall short.