Streptomyces:Other Bits/Useful Molecular and Chemical Equations

Other Bits - Useful Molecular and Chemical Equations
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Useful Molecular and Chemical Equations
Formula Weight & Molecular Weight

Formula weight (FW) and molecular weight (MW) are calculated by summing the atomic weights (AW measured in atomic mass units, amu) of the individual atoms.

The difference between formula weight and molecular weight depends on the compound. It is correct to refer to a compound such as Benzene having a molecular weight or formula weight. It is incorrect to refer to sodium chloride having a molecular weight as NaCl exists as an ionic compound (Na+ Cl-) not as a molecular compound. In this case it is more precise to refer to sodium chloride’s formula weight.

Avogadro’s Number & mole

1 mole of atoms / molecules has a mass equal to the atomic / molecular weight in grams.

e.g. 1 mole (1mol) NaCl is the number of molecules in 58.44g of NaCl. (1mol NaCl = 58.44g)

Avogadro’s number is the number of atoms / molecules in 1 mole of any substance, which is equal to 6.02214x1023.

$$n=\frac {m}{FW}$$

Molarity – Molar Concentration

Molarity is the number of moles of solute per litre of solution.

e.g. 6 molar (6M) HCl is equal to 6 moles (6mol) of HCl per litre (L). (6M HCl = 6mol/L)

$$M=\frac {n}{V}$$

Based on the previous two equations:

Primer Calculations

Primers are dissolved in sterile distilled water (sdH2O) to a concentration of 500pmol. Use one of the following to determine what volume of sdH2O to use:

Weight / mole Percentage

The percentage weight of an element in a compound is calculated using the atomic weight and formula weight.

$$\frac {AW}{FW}*100=%weight$$

Percentage weight of Cl in HCl:

$$Cl=\frac {35.45}{36.45}=97.26%$$

Similarly, mole percentage is a ratio.

$$\frac {x}{T}*100=%mole$$

Percentage mole of Cl in HCl:

$$Cl=\frac {1}{2}*100=50%$$

Density and Specific Gravity

Density is the mass of a substance per volume.

Specific gravity is a unitless ratio, so for all purposes; SG &#8801; D. Cubic centimetres are equivalent to millilitres; cc &#8801; mL.

D2 = Density of H2O @ 4&#176;C = 1.00g/cc

Molarity, Specific Gravity and Percentage Composition

Calculating Molarity from specific gravity and percentage composition:

Percentage composition means xg of pure compound per 100g of solution, i.e. 37g/100ml = 37%. To calculate the molarity, the mass of pure compound is needed; however the solution’s specific gravity needs to be taken into account, and the volume; which we’ll take to be 1L.

Where 1L = 1000cc For HCl:

$$m=\frac {1.18*1000*37}{100}=436.6g$$ Therefore: $$M=\frac {436.6}{1*36.45}=11.97mol/L$$


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