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# Making BL21 competent cells

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### #1 biology_06er

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Posted 12 February 2012 - 01:33 AM

Hi there,

I'm making competent cells for the first time and I have to use 20mM Mg2SO4 and 10mM KCl so I'm just wondering how to make these solutions. I assume I make 1M solution of each and to do so I would do the following:

To make 1M Mg2SO4 I would dissolve 144.673g of Mg2SO4 in 1000mL of water but seeing as thats too much if I wanted to 100mL I would dissolve 14.5g in 100mL of water?..Is this correct?

To make 1M of KCl I would dissolve 74.55g in 1000mLs of water but once again 1000mLs is too much so if I wanted to make 100mL i would dissolve 7.4g in 100mLs of water...is this correct?

Cheers,
b_06er

### #2 bob1

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Posted 12 February 2012 - 01:10 PM

They are correct (assuming you got the molar masses correct) - if you use the following two formulae: Concentration =Number of moles/Volume (C=N/V) and Number of moles = mass/molar mass (N=M/Mr) you can't go wrong.

### #3 mdfenko

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Posted 14 February 2012 - 08:02 AM

since you are rounding to the nearest tenth gram, kcl should be 7.5gm
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