For the decomposition of sodium bicarbonate (NaHCO3) into sodium carbonate (NaOH), and carbon dioxide (CO2) what is the overall enthalpy of the reaction (ΔHrxn)? Is it endothermic or exothermic?
ΔHf NaHCO3 = -950.8 kJ
ΔHf NaOH = -425.6 kJ
ΔHf CO2 = -393.5 kJ
ΔHf Na2CO3 = -1130.7 kJ
ΔHf H2O = -285.8 kJ

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A3.37-mg sample of protein was chemically digested to convert its nitrogen into ammonia and then diluted to 100.0 ml. then 10.0 ml of this solution was placed in a 50-ml volumetric flask and treated with 5 ml of phenol solution plus 2 ml of sodium hypochlorite solution. the sample was diluted to 50.0 ml, and the absorbance at 625 nm was measured in a 1.00-cm cuvette and found to be 0.486. for reference, a standard solution was prepared from 10.0 mg of nh4cl (molar mass = 53.49 grams/mole) dissolved in 1.00 l of water. then 10.0 ml of this standard was placed in a 50-ml volumetric flask, treated in the same manner as the unknown, and the absorbance found to be 0.323. finally, a reagent blank was prepared using distilled water in place of unknown, it was treated in the same manner as the unknown, and the absorbance found to be 0.076. calculate the weight percent of nitrogen in the protein.

Significant figures are digits read directly from the measuring instrument plus one more digit, which is __ by the observer.