Chemistry 115 and Chemistry 116
Answer Formatting in WebCT
Important: If you close the main WebCT webpage before submitting your answers or homework, your homework will be listed as ungraded. You must click Submit, Finish, OK, View Results for your homework to be graded. Your homework must have been graded for you to be able to go back and review your questions and answers.
WebCT does not recognize certain functions in the answer fields of the program and the following guidelines will be used for entering answers unless directed otherwise.
CHEMICAL FORMULAS IN WebCT Answers
1) When entering chemical formulas in answer fields, the subscript and superscript fonts are not available.
For molecular formulas: H2SO4 must be entered as H2SO4 (just as you would read the formula left-to-right but without subscripts); the formula for ammonium phosphate would be entered as (NH4)3PO4. The number “1” is not used as a subscript since it is implied as soon as you write the symbol for the element in a formula.
For ions alone: SO42 - must be entered as (SO4)2- (parentheses around the chemical composition followed by the charge, number first then sign). The number one is omitted. For example: the formula for the ammonium ion would be entered as (NH4)+; the formula for the nitration ion would be entered as (NO3)- ; the chloride ion would be (Cl)-; the sulfide ion would be (S)2- , the zinc ion would be (Zn)2+, etc. )
For hydrates: Use an asterisk, with no spaces, to separate the formula of water from the other component. For example, the formula for cobalt(II) nitrate hexahydrate would be entered as Co(NO3)2*6H2O.
2) Watch your zeros and "O's" and well as spaces. Entering H20 or H2 O instead of H2O for water will be counted as a wrong answer.
NAMES OF COMPOUNDS and IONS in WebCT Answers
1) Spelling and spacing are important. If an answer is not spelled correctly it is wrong. Spelling of the names of elements and compounds will follow the conventions in your textbook. For example, if you enter flourine instead of fluorine, then your answer will be counted as a wrong answer.
2) Enter your answers for names of elements, compounds, and ions using all lower case letters. For example, oxygen, calcium nitride, carbon dioxide, sulfate ion, copper(II) ion.
3) For names of ions where you need to show the ion charge: no space between the symbol of the element and its ion charge which is in parentheses. For example: the name of the Co2+ ion is entered as cobalt(II).
CALCULATING MOLAR MASS (or MOLECULAR WEIGHT) VALUES and ENTHALPY, FREE ENERGY, AND ENTROPY VALUES FOR CHEMICAL SYSTEMS
All molar mass values used in the computations for the problem answers were calculated using atomic mass values given in the Periodic Table on the inside front cover of your textbook. The computation of molar mass values is based on the math concept of addition…NOT multiplication. [Expressing (79.90 g/mol + 79.90 g/mol) as 2(79.90 g/mol) is merely a simplification in writing format.] Therefore, the significant figure rules for addition and subtraction apply to calculation of molar mass values and are based on number of digits after the decimal point.
The same principles of addition and subtraction apply in thermochemistry (Chapter 6) and thermodynamics (Chapter 20) when two or more equations are added and/or subtracted in order to obtain the enthalpy (Hess’s Law), free energy, and/or entropy for some overall or combined reaction.
BALANCED CHEMICAL EQUATIONS
Generally chemical equations are balanced using smallest integer values as coefficients.
Exceptions: In thermochemistry (Chapter 6) and thermodynamics (Ch 20), balancing an equation so that the substance of interest is expressed as 1 mole is common. Therefore, fractions are often used to balance thermochemical equations. For example: H2(g) + 1/2 O2(g) à H2O(g)
NUMERICAL ANSWERS
1) Unless instructed to do otherwise, it is advisable to enter your answers in scientific notation (format: #.##E##). For example: 3.45E34, 3.4E-3, 3E3. In other words, replace "x 10" with E, for example, 0.00345 becomes 3.45E-3 in WebCT.
If standard decimal notation is required, include any leading zeroes. For example enter 0.123, not .123 to be consistent with the format in your textbook. For very large numbers such as 2,500,000 enter the commas if standard decimal notation is required, otherwise enter in scientific notation as shown above.
2) Don't forget to use the correct numbers of significant figures in your answers for all homework assignments.
3) Special note about special significant figure cases:
For a temperature value given with a zero in the one’s place, e.g., 20 oC , the zero is normally considered to be a significant digit. Therefore, 20 oC would be precise to 2 significant figures (the “2” as well as the “0” in the one’s place.), not 1 significant figure. Changing 20 oC to K increases the number of significant digits by 1 [(20 + 273) = 293 K] for computation purposes..
760 torr (= 1 atm) is considered to have 3 significant digits.
CONSTANTS
Values for constants will be found in your textbook. Use the value given, don’t round off for your computations.
UNITS
1) The answer fields labeled as “units” are where the appropriate unit abbreviation must be typed. Most of the time these abbreviations are the same as what you will find in your Silberberg textbook, for other situations use the units indicated in the problem set or those in the table below.
2) When entering units, just enter the accepted abbreviation for that unit. The unit for 18 grams of H2O is entered as g; for 1.0 mol H2O, enter the unit as mol.
3) If a unit box does not appear after the question, units are not required.
4) Units are case sensitive.
Area = m2, cm2, in2, etc / Hertz = Hz / Molar mass = g/molAtomic/isotopic mass = amu / Hours = h / Molar solubility = mol/L
Atmosphere = atm / Joules, etc... = J, kJ / Moles = mol
Becquerels = Bq / Joules/mole = J/mol, kJ/mol / Molecules = molecules
°C =degrees C / Kelvin = K / Moles per liter per second = molL-1s-1
Concentration = M / Liters, etc... = L, mL / Particles = particles
Coulombs = C / Meters, etc... = m, cm, km / Percent = %
Curies = Ci / Meters per second = m/s / Pounds = lbs
Days = day / Metric tons = metric tons / Seconds = s
Density = g/mL / Miles = mi / Per second = s-1
Gallons = gal / Miles per hour = mi/h / Torr = torr
Grams, etc... = g, kg, mg / Minutes = min / Volts = V
Molality = m / Molarity = M / Years = yr