1. What is the mass of the solute contained in 21G. solution with a mass fraction of 0.9% 2. How much water should be added to 24 g of salt to get a solution with ω=23 %?
3. What mass of salt should be added to 85 ml of water to get a solution with ω = 17 %
4. Mixed 1.2 g of salt and 2.4 ml of water. What is the mass fraction of salt in the resulting solution?
5. To 36G of 19% salt solution, 21 ml of water was added. Determine the mass fraction of salt.
6. To 85g of a 13% strength solution of sugar added 14g of sugar. What is the percentage of sugar in the newly obtained solution?
7. How many milliliters of water should be added to 54 g of 18% salt solution to get a 5% solution?

since the atomic number is the number of protons in the nucleus, as we move to the left on the periodic table, the electrostatic pull between the nucleus and the valence electron shell decreases. as we move down a group (vertical column), the valence shell energy level gets further and further from the nucleus.

1. the question asks the most appropriate, which seems to rule out "all of the above." your choice appears to be the correct one.

2. the protocol described is good for identifying correlation, but not cause and effect. for cause/effect, you need to devise a protocol that will assign the treatment (or not) to groups that have essentially the same "demographics" with respect to all the other variables. your choice regarding control group seems appropriate. random assignment of treatment (choice d) may be appropriate also. (i think you can argue that b and d are just different descriptions of the same method.)

3. d will randomize the birds, but may not guarantee there are 10 in each group. e will do both, so seems the better choice.

comment on question 2

based on the problem statement, it seems clear that confident, attentive drivers are more likely to use daytime running lights. so, the protocol might need to be devised in such a way that the lights would be turned on or off without the driver's knowledge. (an attentive driver will figure it out.) assigning a confident, attentive driver to a group required to drive without running lights might mess with his/her confidence and attentiveness. you don't want your experiment to do that. (nor do you want the experiment to subject a driver to greater risk of accident as a consequence of driving without running lights.) experiments involving human safety are always problematic.

i just learnt this recently but i need to look through my notes again

a