3. The two most abundant isotopes of copper contain 34 and 36 neutrons. If the isotope with 34 neutrons comprises 72.5% of all copper, what is the average atomic mass of copper?

63.55 amu

Explanation:

The average atomic mass of an element can be calculated by multiplying the decimal abundance of each isotope of that element by the relative atomic mass and summing them up. That is;

Average atomic mass = {atomic mass of isotope 1 × decimal abundance of Isotope 1} + {atomic mass of isotope 2 × decimal abundance of isotope 2}

According to this question, two most abundant isotopes of copper contain 34 and 36 neutrons. The atomic mass of each isotope can be found by adding the no. of neutrons to the atomic number/proton no., which is 29.

Atomic mass of Isotope 1= 34 + 29 = 63

Atomic mass of Isotope 2= 36 + 29 =65

Decimal abundance of each isotope = percentage abundance ÷ 100

Isotope 1 (with 34 neutrons) = 72.5% = 72.5/100 = 0.725

Isotope 2 (with 36 neutrons) = 100 - 72.5 = 27.5% = 27.5/100 = 0.275

Therefore, the average atomic mass

= {63 × 0.725} + {65 × 0.275}

= {45.675} + {17.875}

= 63.55

Hence, the average atomic mass of copper is 63.55 amu

cells use diffusion to get substances in and out of themselves. that diffusion will occur across the cell membrane or the outer covering of the cell. because we are talking about the surface of the cell, surface area and volume are  important factors here. as a cell increases in size, its surface area and volume increase as well.  we call that the surface area to volume ratio (sa: vol).  but the catch is that  volume increases at a greater rate than surface  area. in other words, as a cell gets bigger, the sa: vol gets smaller. so a really small cell like bacteria is mostly cell membrane (surface) with very little contents (volume). a really big cell like an egg, is mostly insides with a super thin membrane. so what ends up happening is that the cell cannot efficiently diffuse all of the stuff it needs in/out to survive. therefore cell size is limited to about 100mm. so how do we make organisms bigger than 100mm? cells divide! by dividing and becoming multicellular, organisms are able to grow!   check out the diagrams i attached to see the effect of sa: vol. i hope this ! good luck!