EXAMPLE: 7A: The maximum amount of magnesium fluoride that will dissolve in a 0.
ID: 1009735 • Letter: E
Question
EXAMPLE:
7A:
The maximum amount of magnesium fluoride that will dissolve in a 0.200 M potassium fluoride solution is Incorrect KF is a soluble salt, MgF2 is not -3 The solubility of MgF2 in pure water is 1.2×10-3 M. The amount of magnesium fluoride that can dissolve in a solution of potassium fluoride is less than in plain water, because there is already fluoride ion present in the solution. This is known as the "Common Ion Effect" Initial [F]-0.200 M from the KF solution Step 1: Set up the equilibrium using the ICE method Let 's'- number of moles of MgF2 (s) per liter that dissolves MgF2 (s) Mg M2+ (aq) + 2F~ (aq) 0.200 M + 2s 0.200 + 2s Initial some Change Equilibrium some Setp 2: Substitute the equilibrium values into the Ksp expression Ksp =[Mg2+] [F]2-6)(0.200+ 2s)2 = 6.4× 10-9 (from the table) Step 3: Assume that 2s is small relative to 0.200 M, so that the approximation 0.200 + 2s 0.200 can be made. This is reasonable because the solubility is low without the common ion and it will be even lower in the presense of added Ksp (s(0.200)2 -6.4x10 F. Then: Step 4: Solve for s 6.4x10" 1.6x10-7 M (0.200) Step 5: Check the approximation 0.200+ 2s 0.200+2(1.6x10") = 0.200-0.200 Previous Next OKExplanation / Answer
is prepared from magnesium oxide with sources of hydrogen fluoride such as ammonium bifluoride:
MgO + (NH4)HF2 MgF2 + NH3 + H2O
Related metathesis reactions are also feasible.
The compound crystallizes as tetragonal birefringent crystals. The structure of the compound is similar to that in rutile, featuring octahedral Mg2+ centers and 3-coordinate fluoride centres.[2]
Uses[edit]
Optics[edit]
Magnesium fluoride is transparent over an extremely wide range of wavelengths. Windows, lenses, and prisms made of this material can be used over the entire range of wavelengths from 0.120 m (vacuum ultraviolet) to 8.0 m (infrared). High quality synthetic VUV grade MgF2 is quite expensive, in the region of $3/kg (2007) but the real cost of optics in this material is due to relatively low volume manufacture. However, with lithium fluoride it is one of the two materials that will transmit in the vacuum ultraviolet range at 121 nm (Lyman alpha) and this is where it finds its application. Lower grade MgF2 is sometimes used in the infrared but here it is inferior to calcium fluoride. MgF2 is tough and works and polishes well, but it is slightly birefringent and should be cut with the optic axis perpendicular to the plane of the window or lens.[2]
Due to its having a suitable refractive index of 1.37, thin layers of MgF2 are very commonly used on the surfaces of optical elements as inexpensive anti-reflective coatings.
The Verdet constant of (MgF2) at 632.8 nm is 0.00810 arcmin G1 cm1
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