The X-ray crystal density method is primarily a method for determining the Avogadro constant NA but as the Avogadro constant is related to the Planck constant it also determines a value for h. The principle behind the method is to determine NA as the ratio between the volume of the unit cell of a crystal, measured by X-ray crystallography, and the molar volume of the substance. Crystals of silicon are used, as they are available in high quality and purity by the technology developed for the semiconductor industry. The unit cell volume is calculated from the spacing between two crystal planes referred to as d220. The molar volume Vm(Si) requires a knowledge of the density of the crystal and the atomic weight of the silicon used. The Planck constant is given by
If it is possible to answer this question, answer it for me (else, reply "unanswerable"): Calculating molar volume requires knowledge of what two factors?
Ah, so.. the density of the crystal and the atomic weight of the silicon used

In the example above, the identity and the rotations constitute a subgroup R = {id, r1, r2, r3}, highlighted in red in the group table above: any two rotations composed are still a rotation, and a rotation can be undone by (i.e. is inverse to) the complementary rotations 270 for 90, 180 for 180, and 90 for 270 (note that rotation in the opposite direction is not defined). The subgroup test is a necessary and sufficient condition for a subset H of a group G to be a subgroup: it is sufficient to check that g1h H for all elements g, h H. Knowing the subgroups is important in understanding the group as a whole.d["]
If it is possible to answer this question, answer it for me (else, reply "unanswerable"): 270 for 180 is an example of what kind of rotation?
Ah, so.. unanswerable

One of the more prominent contestants this year was Chris Medina, whose story of caring for his brain-damaged fiancee received widespread coverage. Medina was cut in the Top 40 round. Casey Abrams, who suffers from ulcerative colitis, was hospitalized twice and missed the Top 13 result show. The judges used their one save on Abrams on the Top 11, and as a result this was the first season that 11 finalists went on tour instead of 10. In the following week, Naima Adedapo and Thia Megia were both eliminated the following week.
If it is possible to answer this question, answer it for me (else, reply "unanswerable"): Which contestant had a fiance that had suffered brain damage?
Ah, so..
Chris Medina