The determination of metal ion in a water sample was carried out with atomic emission spectroscopy and the method of standard addition. The instrument was autozeroed before measurements. Two identical portions (50 mL each) of the original sample were taken. One portion was measured directly and emission lines at 282.3 nm and 589.6 nm were detected. To the other portion, 20 μL of a 10.0 μg/mL NaCl standard was added and the resultant solution's emission intensity at 589.6 nm changed from 349 to 698, while its emission at 282.3 nm remained at 250. What was the sodium concentration (unit: μg/mL) in the original water sample? (Chlorine does not emit at 282.3 nm, nor at 589.6 nm.)

The last few miles of the marathon are the most difficult for heather, her hair plastered to her head, sweat clinging to her arms, and her legs already feeling as if they had nothing left, just dead weight. after grabbing a cup of ice water, she feels the ice cubes smash against her nose as she gulps some cool refreshment and keeps on running. in these last few miles, the breeze kicks up and she finally feels some coolness against her skin. drips of sweat, once clinging to her forehead, now spill down, and heather feels more pain as the sweat flows into her eyes.which of the following is the most likely reason why the ice struck heather’s nose when she took a drink? a) water can function as a solvent. b) water can store large amounts of heat. c) water can moderate temperatures through evaporative cooling. d) the density of water decreases when it freezes. e) water has a cohesive nature.sweat remained on heather’s forehead and arms because of the a) high salt content of sweat b) cohesive nature of water c) ability of water to moderate heat d) high evaporative cooling effect of water e) ability of water to act as a solvent