SC190 Unit 3 equilibrium calculation, finished: two ICE tables, a failed small-x check and one disturbance, settling HI at 0.787 M and NO2 at 0.0204 M. Searches like "sc 190 unit 3 assignment example", "sc190 unit 3 sample" and "sc190 unit 3 example" land here.
What a finished SC190 Unit 3 equilibrium calculation looks like
Three worked problems, tables drawn in full. For H2 plus I2 forming 2HI, the change row reads minus x, minus x, plus 2x, and taking the square root of both sides of 54.3 equals (2x) squared over (0.500 minus x) squared gives x equal to 0.393, leaving 0.107 M each of hydrogen and iodine against 0.787 M HI. Substituting back returns 54.3. For N2O4 forming 2NO2 with Kc of 4.63 x 10^-3 at 25 degrees, the small-x approximation gives x equal to 0.0108, which is 10.8 percent of 0.100 M and over the 5 percent limit, so the quadratic is solved: x equals 0.0102 and NO2 is 0.0204 M. The third problem adds 0.200 M hydrogen to the settled first system, computes Q as 18.9, below K, and shows the shift to the right.
How a SC190 Unit 3 example is structured
Each system is handled in five moves: write the balanced equation and the K expression, build the ICE table with the change row scaled by the coefficients, substitute into K, solve, and check. The check is written out, never implied, with equilibrium concentrations placed back into the expression to confirm they return the stated K. Where an approximation is used, the 5 percent test is stated with its numbers before the answer is accepted. The disturbance problem opens with a qualitative prediction from Le Chatelier's principle and then confirms it by comparing Q with K, so the direction is argued twice from different grounds. Its second ICE table starts from the disturbed concentrations, not the original ones, and settles near 0.887 M HI. A closing paragraph explains in words what the size of K says about how far each reaction goes.
Coefficients carried into the change row
The plus 2x under hydrogen iodide is the line most sets get wrong, and the calculation labels it as the stoichiometric ratio applied to the change rather than leaving it unexplained.
Square roots where the algebra allows
Equal starting amounts make both sides of the first K expression perfect squares, so the calculation takes a square root instead of expanding into a quadratic, and notes why the shortcut is legitimate there.
The 5 percent test, with numbers
For N2O4, 0.0108 over 0.100 is 10.8 percent, and the calculation says so before rejecting the approximation, so the quadratic that follows is justified rather than habitual.
Q against K for the disturbance
Adding hydrogen drops Q to 18.9 while K stays 54.3, so the system must make more product. The prediction from Le Chatelier's principle and the arithmetic agree.
Back-substitution as proof
Each set of equilibrium concentrations goes back into its K expression, returning 54.3 and 4.63 x 10^-3, the check that confirms no algebra slipped along the way.
Where marks go in SC190 Unit 3
The change row is where most marks disappear: x written under HI instead of 2x, which makes every later line wrong while looking tidy. Approximations used without the 5 percent test come next, and the dinitrogen tetroxide problem exists partly to catch them. Solids or pure liquids included in K expressions, or products and reactants inverted, cost accuracy credit at the first line. Disturbance answers that state a direction without comparing Q and K, or that change K itself when only a concentration changed, lose the reasoning marks the unit emphasizes. Second tables built from the original starting amounts rather than the disturbed equilibrium give answers that fail their own check. Missing units on concentrations, K carried to more figures than the data allow, and a skipped back-substitution account for the smaller deductions.
Get a SC190 Unit 3 example written to your instructions
Type out the Unit 3 systems as the SC190 assignment gives them, with starting amounts, K values, temperatures and the rubric. Each system is set up as an ICE table, solved with any approximation tested and checked by substitution, with disturbances argued by Q against K. A first sample is free and yours within 24-48h.
SC190 Unit 3 questions, answered
When is the small-x approximation allowed?
When x turns out to be less than about 5 percent of the starting concentration it was subtracted from. The approximation has to be tested after solving, not assumed before. It usually holds when K is very small compared with the starting concentration, and it fails, as with N2O4 here, when K is large enough that a noticeable fraction reacts.
Does adding a reactant change the equilibrium constant?
No. Only a change in temperature changes K. Adding hydrogen changes Q, the same expression evaluated with current concentrations, and the system shifts until Q equals K again. Many lost marks in Unit 3 come from answers claiming K increased after a concentration change, which contradicts the definition the whole calculation depends on.
What does the size of K tell me?
Roughly how far the reaction goes before settling. A K of 54.3 for hydrogen iodide means products are favored at equilibrium, which the table confirms with 0.787 M HI against 0.107 M of each reactant. A K of 4.63 x 10^-3 means reactants dominate, and only about a tenth of the N2O4 dissociates.