A plot of the concentration of nitrogen trioxide versus time with a slope of 0.260 gives a straight line. What was the initial concentration of nitrogen trioxide if after 35 seconds its concentration dropped to 2.75×10-2 molar? All right, so here they're telling us it's a plot of concentration versus time. Remember, if our plot is of your reacting concentration versus time, that would mean that it is a zeroth order reaction or zero order reaction, so we know it's zero order.
So that means that our final reacting concentration equals negative KT initial reacting concentration. Now here they're asking us to figure out the initial concentration. So we don't know what this portion is. We know that it drops to this final number here. So that's our final concentration. So that's 2.75×10-2 equals. Now remember this equation for zeroth order reaction is also equal to the equation for a straight line, and here is our slope which is equal to our negative K.
So when they tell me my slope is 0.260, they're really telling me what K is. So we're going to plug that in for K. So negative 0.260 T here is time, which we're told is 35 seconds. All right, so then we're going to have 2.75×10-2 = -9.1 plus the initial concentration of your reactant. Here you're going to add 9.1 to both sides, and when we do that, we're going to get as our initial concentration 9.12 75 molar.
So this would represent our initial concentration here in our question. This has three sig figs, 3 sig figs, 2 sig figs. So here if we had it in terms of two sig figs, it would come out to be 9.1 molar as our initial concentration, right? So that would be our final answer.