"Starting with this speech, explain how far you think Shakespeare presents Lady Macbeth as a powerful woman."

This is a close-reading question from the 2014 AQA paper on 'Shakespeare and the 19th Century novel'. The actual question is for 30 marks in an exam, so this is a shortened version of what one might write.
This speech presents the character Lady Macbeth as a powerful character by making use of feminine language and imagery and subverting it to make her sound cruel and vicious. For example, she wills the "spirits" to "take my milk for gall". That is, she asks for demonic, supernatural spirits to replace the milk in her breasts with gall (bile). Gall is bitter and dark, and is symbolically replacing the nourishing and sweet milk that she should naturally produce. Gall is dark-coloured, and this is also symbolic of Lady Macbeth's turn towards darkness, away from the 'pure' and 'innocent' white of milk. In this way, Lady Macbeth is presented as someone who takes her natural femininity, (and the associated vulnerability and gentleness that this connotes),and turns it into something dark and powerful.
However, just as this subversion of her natural femininity might be seen as presenting Lady Macbeth as powerful, it perhaps also suggests a weakness. Lady Macbeth's character is not, as one might expect from more contemporary feminist characters, someone who embraces her role as a 'woman' and takes advantage of it. Rather, she sees it as an obstacle to be overcome: this is demonstrated by her asking the spirits to "unsex me here". She is here asking the spirits to remove her sex - to make her stop being a woman that she might be more capable of power and viciousness as though she were a man. Moreover, the very fact that she is asking spirits to intervene here to help her perhaps shows an underlying tone of vulnerability - Lady Macbeth is not able to use her nature and natural capacities to determine her actions for herself; she sees her own nature as something to be overcome by external forces.