Answer:
<em>weight of H₂O formed = 2.3 grams H₂O (2 sig. figs.)</em>
Explanation:
Rxn: CH₄(g) + 2O₂(g) => CO₂(g) + 2H₂O(g)
Given: 1.16g 4.16g 3.52g ? (g)
Moles: 1.16g/16g·mol⁻¹ 4.16g/32g·mol⁻¹ 3.52g/44g·mol⁻¹
= 0.0725mol. = 0.13mol. = 0.08mol. => ? (moles)
Limiting Reactant: Divide each mole value by related coefficient of balanced standard equation (that is, balanced with coefficients in lowest whole number ratios). The smaller value is the limiting reactant.
0.0725/1 0.13/2 0.080/1
= 0.0725 = 0.065 = 0.080
Limiting Reactant is O₂(g) => 0.065 is smaller value after dividing each mole value by related coefficient of balanced equation.
NOTE: When working problem, however, one must <em><u>use the mole value</u></em> calculated from given amount in grams. That is, in this case 0.13 mole O₂. <u>The </u><u><em>'divide by related coefficient and check smaller value' </em></u><u>is ONLY for identifying the limiting reactant. This trick works for ALL general chemistry problems.</u>
Moles H₂O formed: Since the coefficient of the limiting reactant (O₂) equals the coefficient of water (H₂O), then the moles of water formed is 0.065 mole H₂O.
Weight (in grams) of H₂O formed:
Grams H₂O = moles H₂O x formula weight H₂O
= 0.13 mole H₂O x 18 g H₂O/mole H₂O
= 2.34 g H₂O (calculator answer)
= 2.3 g H₂O (final answer should be rounded to 2 sig. figs.) => form of final answer should be based on data in <em>final computation </em>having <u>the least number of sig. figs. </u>
Review: Sequence of calculations
- Write and balance equation to smallest whole no. ratio of coefficients.
- If not in moles, convert given 'measured' data to dimension of moles. => moles = mass (g)/formula wt(g·mol⁻¹) => moles = volume of gas in Liters/Std Molar Volume (= 22.4L·mole⁻¹ at STP) => moles = no. of particles / Avogadro's No. (= 6.02 x 10²³ part's/mole)
- Determine Limiting Reactant => mole values of each compound given / related coefficient in standard equation => smallest value is L.R.
- Determine moles of unknown needed/used/formed from limiting reactant in moles and coefficient of unknown compound in standard equation given data values.
=> moles of limiting reactant / coefficient of same cpd. in std. equation = unknown (X) / coefficient of same (unknown) cpd. in std. equation
=> cross multiply and solve for unknown (X)
=> L.R.(moles) / eqn. coef. of L.R. = X / eqn. coef. of X
=> (L.R.(calc'd moles)(eqn. coef. of X) = (X)(eqn. coef. of L.R.)
=> X (in moles) = (L.R.(calc'd. moles)(eqn. coef. of X) / (eqn. coef. of L.R.)
- Convert X-answer in moles to desired dimension specified in problem.
=> grams = moles x formula wt.
=> volume (L) = moles x std. volume (= 22.4L/mole)
=> #particles = moles x Avogadro's Number (= 6.02 x 10²³ parts/mole)